How are his claims not true? Mozilla posted Metalab's designs on a public wiki page and used them for promotion - after rejecting the design (ie. not paying for it). If you don't want to pay for it, don't use it. That's pretty straightforward ethical behaviour, I would've thought.

MetaLab did no work for Mozilla beyond giving them an estimate for work to be completed. The concept drawing the Mozilla mistakenly posted on their wiki was, by all accounts, a concept that was produced internally at Mozilla.

An estimate that Mozilla rejected before using the design they were unwilling to pay for.

EDIT: If I were MetaLabs, I would not be horrified at the prospect that people perfectly willing to rip me off would be reluctant to hire me.