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.

ASCII is a 7-bit encoding. That’s it.

Honestly I’m not even convinced it’s entirely fair to call code page 437 a superset of ASCII, with how it repurposes the control codes 0x01–0x1F and 0x7F. (Superset of printable ASCII, definitely. It just feels not quite right to call it a superset of ASCII as a whole, though it is generally considered so.)

(And I can’t find any references to code page 437 being called “IBM PC ASCII”.)