But the EUPL itself says that derivative works may be distributed under the LGPL instead, as long as those works are derived from both a EUPL and LGPL parent. This seems kinda trivial to abuse and the author should have noticed this clause if that was their main concern.
Huh, that's a pretty massive loophole. Pretty much makes the whole license irrelevant.
The EUPL obligations still apply, unless they specifically conflict with the other license. There is some dispute on how exactly that is to be interpreted, but hopefully it will be clarified in version 1.3: https://interoperable-europe.ec.europa.eu/collection/eupl/di...
A “conflict” here is intended to mean that following some EUPL obligation (such as publishing the derived source code) wouldn’t be allowed under the alternative license.