Just because it’s legal and allowed doesn’t mean it’s not off putting.

Personally, I have no issue with them making their own internal fork, but then blogging about their thing without contributing it back leaves a little bad taste. If it’s so good, then contribute it back, since they benefited from the volunteers.

You can't have it both ways. As a library author choose MIT to encourage commercial usage because companies are afraid of GPL, but then complain that companies are actually using it in a MIT license way without contributing back.