The OP was joining a conversation about the shape of the Earth. It maybe could have started with, "Many flat Earthers support the idea of a disk with an edge. However, if..."
But, it didn't. Did it?
So when you reply, "If it were infinite...," I don't think that it's too much of a stretch to assume that you were offering a counter-explanation or alternate theory. This is a forum for discussing/debating the shape of the Earth, is it not?
Did you reply to the OP?
No. I replied to replies to the OP. Did you ever say anything substantive other than suggesting the "infinite Earth" hypothesis (and then pretending that you didn't)?
Honestly, I don't think that it's the best debunking of FE because it would (predictably) be followed by conspiracy nonsense (as we saw).
This is my favorite: The most common FE model has a flat Earth with a diameter of about 25,000 miles and a Sun circling 3,000 miles overhead while lighting up half the Earth. If that's the case, then the Sun would never be lower than about 20 degrees. (This is best if the person knows a little basic trigonometry so they can calculate inverse tangent, but it is not absolutely necessary.) Perspective would never get the Sun close to the horizon if it is that high. (Well, it could, but it would have to move far beyond the dimensions of the Earth.)