I came to this sentiment a few years ago when I first switched over to Jellyfin and was figuring out metadata providers and investigating the -arr services.
To be fair to Sonarr, I never found any kind of communication where they explicitly said "TMDB will never be supported" so I shouldn't have said so explicitly "Sonarr has made it clear", but a lot of the responses to support about TMDB that I came across had included suggestions like "stop using Sonarr", "Sonarr is free so enjoy what you're getting", and "if you want a feature, add it yourself". These answers, while technically valid, had a pretty stubborn tone, at least to me.
Also, while the people providing these kinds of answer obviously don't necessarily represent Sonarr as a whole, they did claim to be active supporters, and I found enough of them at the time that it soured my impression of Sonarr.
I think I may have found a few of them from back then:
https://www.reddit.com/r/sonarr/comments/nqe05n/any_way_around_tvdb/
https://www.reddit.com/r/sonarr/comments/l046yc/trakt_is_switching_to_tmdb_as_the_primary_data/
https://www.reddit.com/r/sonarr/comments/ljapuq/pulling_shows_from_tmdb/
https://www.reddit.com/r/sonarr/comments/pmgmbo/will_sonarr_switch_to_or_at_least_provide_an/
https://forums.sonarr.tv/t/tmdb-for-tv-show-information/3624
Edit: As for why they won't support TMDB, some of their apparent reasons are discussing in the above posts, but I don't know how relevant or valid they are today, or even if they were back then.
Who made this comic?