> concern that fragments of GPL code would end up in projects with non-GPL licenses
Framing it like this makes it sound like a problem with the GPL in particular, when really it's a problem for anything (including ingested code with no discernable license) that isn't expressly available under permissive licenses--more permissive than even the BSDs or MIT license, which after all, still require attribution by way of reproducing any copyright notices and distributing the text of the license itself.
The only terms where this isn't really a problem is limited to basically WTFPL and public domain code.
> Are there any AAA game engines released under a BSD license to compare with?
Er, stating what should be the obvious here, but the fact that the answer to this is "no" is really making the point of the person you're challenging...
An more relevant question is, "Are there any commercial licensees of id's code, who would have not had to license it had it been available under a permissive license?" Answer for that is, "Yes, lots".
https://i.reddit.com/r/itrunsdoom