Reasonable assumptions are a fundamental requirement for communication. It’s not that you are wrong in what you are saying. There is a chance that the poser of the question made a visual representation of the triangle’s sides appear to be complementary and appear to construct a straight line across their bases while not actually definitively indicating them as such.

The way these triangle’s are represented is already skewed so perhaps that is what they are trying to do.

The thing is though, at that point they are defying convention and reasonable assumptions so much that they aren’t worth engaging seriously because it’s flawed communication.

The version people are choosing to answer seriously is equivalent to a guy holding up a sign that says “ask me about my wiener to get one in a flash for free!” while standing next to a hot dog stand. If you ask he flashes his junk at you and says cheekily “haha you just assumed wrong! Idiot!”

That’s already dumb enough but some people could see the clues that suggest he was actually intended to flash people the whole time through a series of reasonable assumptions about his outfit lacking pants or the hit dog stand not even being turned on.

Your argument that we can’t assume the line at the bottom is straight is like saying we can’t assume the theoretical trenchcoat man won’t toss a rabid dachshund he was hiding under the coat at us because the hot dog stand has no buns or condiments on it.

You might not be provably wrong but it’s really not worth thinking like an insane person just because a few conventions were defied