What do you suppose they should do to the original authors? Perhaps the original author is Claude Garamond who died in the sixteenth century? Or the unknown workers who carved the inscription at Trajan's column in the second century AD?
When people say "make completely by themselves", they don't mean that were not influenced by literally no prior works. Artists can say they painted an original work entirely by themselves without needing a pedant to mention that they were probably influenced by prior work.
Clearly nothing based on Latin script can be original, which itself is based on the Etruscan Old italic alphabet, which itself it based on the Euboean alphabet — ergo, any typeface developed after say the 8th century BC is just a silly derivative.
Everything is derivative, there is nothing new under the sun, and your argument proves nothing.
>at a high enough level of abstraction, all creative work is derivative.
Sure, but with fonts you have basically one level between the font they are 'developing' and the one they are copying from. There is work involved, but very little of it is creative work.
So you understand the issue, it's derivative works all the way down.