You guessed right per Robin Kingsland. An IMDb user or users has been doing weird cross-pollination, mixing Sesame Street crew onto Muppet projects they weren't involved in (and vice versa) including Fraggle Rock, tossing Blue's Clues and Barney crew in, some migration to Peanuts specials even, etc.
The Kingsland writing credits outside of Thomas are either the same person or the same mindset. This is why our policy is whenever possible to verify IMDb credits with what's actually on-screen or otherwise reliably sourced (which you did). So good catch.
Yeah, that's why there's a warning about it in our policy and sourcing pages. IMDb can be handy at times but should always be checked with the source, since really except for when actors update their own pages or studios/press agents contribute for current/recent stuff, it's just like Wikipedia except you can't actually see the user history (and it's harder to correct an error than add one). You get knowledgeable film buffs and fans of specific shows mixed with the trolls and nuts. There's no separate fact checking or research, since IMDb staffers just process edits or handle the news/video etc. stuff.
They did finally remove George Lucas from the cast of Follow That Bird at least.
Another good catch. The page was started by one of those odd users (whose other contributions all had to be deleted), and when trying to remake it to fit our standards, I failed to notice they got the capitalization wrong.
No problem, Jonathan - and thanks for the message! I kinda drifted off the Wiki radar over the last several years (mostly popping in if I spotted a typo or such), but starting around the end of last year I've been getting back into it.
And now, I just realized that next Tuesday will mark a whole decade since my first edit. Wow.