Probably being butthurt about OpenMapTiles and MapTiler "stealing" their business? But then again, Mapbox, judging by the buzzword-laden landing pages and focus on "contact the sales team", seems to be catering to the corporate big fishes. (I may be mistaken though).
Another thing is that Mapbox is owned by SoftBank which hasn't had a good year so far and has to recoup WeWTF losses somehow.
And thank you for the link - I wasn't able to unroll the thread in Threader. I'm probably out of the loop if Twitter has broken their API.