I encountered some push back on this in my "code editor from the ground up" post[1]. I think the only reasonable definition of from scratch is:

Does not have domain-specific dependencies.

So a code editor based on ACE or CodeMirror would not be from scratch, obviously, but one that involves writing all of the domain-specific logic would be. Using generic libraries doesn't stop something being from scratch. (In my case Tree-sitter is arguably domain-specific, but an early version did use a hand-coded JavaScript tokeniser in its place.)

[1] https://news.ycombinator.com/item?id=34577246