The letter says there is no problem. How could a letter saying there
was only a cosmetic problem, induce you NOT to buy? The opposite
seems more reasonable.

Why do you think the letter says there is no problem? I think the letter
says:

There's a crack in the foundation.
The previous owner appears to have attempted a repair.
This repair, and any other repairs they might have made, has only
cosmetic effect.

I think the letter is silent about whether the crack is a problem and, if
so, how serious it is.