My bad, it really annoyed me when npm stopped respecting lockfiles years ago so I stopped using it. That's great news that they eventually changed their mind.

However in rare cases where I am forced to use it to contribute to some npm-using project, I have noticed that the lockfile often gets updated and I get a huge diff even though I didn't edit the dependencies. So I've always assumed that was the same issue with npm ignoring the lockfile, but maybe it's some other issue? idk

Well there are other lockfile updates as well, which aren't dependency version changes either. e.g. if the lockfile was created with an older npm version, running npm install with a newer npm version might upgrade it to a newer lockfile format and thus result in huge diffs. But that wouldn't change anything about the versions used for your dependencies.