They sort of are.

I think the confusion arises because the IBM PC ASCII (code page 437) included comprehensive box-drawing characters between hexidecimal character positions B3 and DA. These weren't adopted into Unicode in the same character positions, but the box-drawing characters were definitely part of the commonly understood ASCII character set.

But I think that this ASCII tree editor should have a toggle option for basic vs extended ASCII, by utilising +, -, and | characters.