Granted, this seems like a case that's less malice and more adequately explained by stupidity.
If you have already somebody's resume, GitHub provide clues and maybe a bit of evidence for your assessment of the candidate. Older, busy working software developer? Expect not many projects, possibly an intermittent commit history. Most GitHub projects are just unfinished exercises, so a workng project is unusual, and a clue that the candidate is also unusual. It's not scientific, but it's evidence that you can use, alongside the resume, and any communication with the candidate.
The history of WP itself enforcing the GP is instructive. More recently, Panasonic Avionics has been sued for $100M due to GPL violations.
A WordPress-generated website is likely not a derivative work of WordPress source code. (Which is why Affero GPL exists.)
The further specifics depend on GPL version employed.
My reaction was largely to the poster's self-righteous attitude about "theft", which has mapped to many GPL violators I've encountered.
I would also check they PR, issue and comments, to get some idea about how they work with stranger; what are important to them when they suggest ideas, contribute to existing code base; how they reply to critic or question from project owner, etc.
Also, just my opinion, most of us - software developers - can't live without OSS. We should contribute back when we could. That's why I usually prefer resume with a non empty Github profile than the others.