I never claimed the ground couldn't be seen.
Yes, you did.
You claimed that because the ground is below the level of the tube, it can't be seen.
I then you a simple question, to clarify this, and this was your response:
Good. Discuss it. Answer the extremely simple question you were asked:
If an object is far enough away horizontally, can you see it even though it is below the tube?
No.
Here you are directly confirming that you believe/claim that an object below the the tube cannot be seen.
And here is another quote from you:
As long as the tube was level (i.e. parallel) with the ground and the tube was not in contact with the ground, then based on his argument you would never see the ground regardless of whether the Earth is flat or not.
Correct.
Where you admit that your claim indicates you shouldn't see the ground through a level tube even if it was flat.
Again, this is because your argument relies upon it being an absolute.
Your argument is that because the ground is below, it can't be seen.
As soon as you admit that you can see things below a level tube, your argument for why you can't see the ground on a slope falls apart.
This is because you CAN see objects below you, which, as explained to you repeatedly, and which you repeatedly ignored, YOU HAVE 2 COMPETING EFFECTS.
One effect is that the ground is physically getting lower.
The other is that "visual compression" making it appear higher.
So the question then becomes which one wins.
Simple math easily allows us to determine that, but you seem to hate that math.
So why don't you try explaining just why we can't see the ground through a level tube on a RE or on a downwards slope.
Remember, you have now admitted that you can see the ground even though it is BELOW a level tube, so that is NOT a justification. You need more. You need to show that the downwards slope can overcome the "visual compression".
You had every opportunity to show your side
And he did, and clearly demonstrated you were wrong.
Your turn now.