“We like your solution but did you seriously just use Maven?”

What did they say exactly?

If I'm being honest here, a quite significant part of day-to-day backend JVM software development positions is dealing with the build tool. While I wouldn't outright reject a candidate for not being familiar with the latest standard toolchain (Gradle), I would certainly consider it as one of many factors - Gradle is pretty complicated, yes, but also quite powerful in what it is capable of achieving. It's not as eyebrow-raising as, say, if you sent in a coding challenge compiled using Ant (which would, in fact, be quite a fun discussion to have with a candidate) but it would certainly be something considered.

perhaps, because your work has over complicated it [1].

[1] If you are doing multi-distributions or multi-repo stuff perhaps it's warranted, but those complex build features better be providing serious value to the business to be worth losing 10% of the development workforce's time on build tooling.