They were actually referring to water in it's normal state, like in lakes, and oceans, and so on.
Everyone already knows water put into a centrifuge, or flowing over a waterfall, is NOT level, or will FIND level, in such cases. They're referring to water in normal state, seeking level, and/or being level.
level adjective
Definition of level
1a: having no part higher than another : conforming to the curvature of the liquid parts of the earth's surface

Yes, I'm well aware of their attempt to revise the definition of 'level', specifically to include a magical, non-existent force, which they had already made up to support the original lie, that Earth is a ball speeding through endless space.
When they use a made up 'force', which they always HAVE used, to support their other claims, like the claim that level means level to a ball Earth surface, the whole argument is garbage, built upon more garbage, and so on.
'Gravity' doesn't exist, has no PROOF of existing, cannot be DEMONSTRATED as existing, because it does NOT exist. It is completely made up, start to finish. Simply knowing that any two objects placed near each other, suspended in air, do NOT attract to one another, unless by MAGNETIC force, which DOES exist, makes 'gravity' a complete lie, a made up 'force', which they HAD to make up, in order to 'explain' how a ball Earth speeding through endless 'space' could ever 'hold down' everything to it. That's why they made up 'gravity', as a massive force within Earth, holding down, pulling down from above, all things to Earth.
However, when they ALSO claimed that 'gravity' existed in ALL objects, it became something that we can actually prove true, or prove false. And we've certainly proven it false, over and over again, and we can prove it is false, today, very easily.
They've claimed that 'gravity' within Earth is a 'weak' force, overall, because they had to 'explain' how a tiny insect or bird could ever fly above Earth, to 'overcome the Earth's gravity' which would be 'holding all things down to the surface', and 'pulling all things down to Earth', when ABOVE the surface.
So they had to claim 'gravity' is actually a 'weak' force, but somehow, it can hold down all the oceans to Earth, and hold down buildings that weigh thousands of tons, and all other things, or most of them, anyway.
That is one of 'gravity's' countless contradictions, which show it is completely made up, and like ANYTHING that is made up, the result is a complete disaster, of opposing claims, that cannot be excused away. Like when 'astronauts' are 'floating' in 'space', which supposedly is 'proof of gravity' existing. Except they ALSO have claimed the moon is 'held at a distance by Earth's gravity, and/or the moon's gravity', while claiming the moon is about 250,000 miles away from Earth. But if THAT was true, it would certainly hold astronauts in place, being much, much CLOSER to Earth, than the moon would be, and weigh far less, too.
If you don't realize that 'gravity' doesn't exist, that there's not a shred of proof that it exists, then I can explain all sorts of contradictions, and conflicts, such as that one, and all the other conflicts it has, which would easily destroy any OTHER claim, but you'll never, ever accept it as BEING proof, let alone know that it IS proof.
Logical thinking, common sense, are the ONLY means we have to find the truth, the facts, or lies, and falsehoods. When they claimed there was a 'force', within Earth, within all objects, and they were attracted to each other, based on their mass and density, and we know it would easily be MORE PROVABLE than magnetic force was proven, by only some objects, with magnetic properties. For THIS force, it would be provable with ANY objects on Earth, because THAT is common sense, and logical.
And when we know that objects do NOT attract together, whatsoever, it is again common sense, and logical, to conclude that such a 'force' does NOT exist at all.
Despite having no proof that 'gravity' exists, you dream up things it can do, like make instruments measure 'level to Earth's curvature', which is ALSO made up, and has no proof of existing, just like 'gravity'.
Anyway, how do we know that level can NOT mean 'level to Earth's curvature'? Because we have another instrument, which measures level, and is called a LASER LEVEL. They do not use Earth's surface, or it's atmosphere, to measure for level. They use lights, which are concentrated to a small point, and cast that light outward, over long distances. No matter WHAT the surface below it, wouldn't matter at all.
ANY curved surface, no matter how slight a curve it has, can be measured, same as a FLAT surface can be measured, or any OTHER surface can be measured.
Your excuse that Earth's 'curve' is too 'slight' to measure, is complete BS. We can measure a curve of microns, on surfaces, with our instruments of today. So we can certainly measure a curve of 8 inches over one mile distance, with our instruments, too.
It isn't that we cannot MEASURE for such a curve, it is that there IS no curve at all, to BE measured for!