I think they’re upset that the library was to be released under LGPL or whatever, even though they clearly went and read from it anyway when implementing their thing.

It seems they're pretty directly admitting to referring to the LGPL library while implementing theirs under a different license.

I wonder if they'll have no issues with people directly reading their code while happening to implement the same functionality with a closed license? Or a GPL-style one?

I'm surprised they admitted to it - it's hardly "Clean Room"....