I'll agree with you, but any good PR director will know how to frame Avatar's success in any marketing.

The point of marketing something as silly as "sequel to the #1 box office film" is to attract casual fans. People that already know spiderman won't care, but there's plenty of casual movie-goers who don't care about Spiderman but might care to see the "sequel to the #1 grossing film". Same with Avatar. I really don't know which they would value more, but I genuinely do think they would lean toward the MCU because of its scale.

Not to mention, simply getting the #1 overall grossing film in its own is a PR boost. Hell they could even re-release Avatar on a limited run to let it try to earn another $100mil or something and then let it reclaim the #1 stake, even more free publicity and now for both IPs.

Doing it that way would even let both films hold their own #1 title. Disney would market MCU films or Avengers sequels as "the long awaited sequel to the top-grossing first-run film", and market Avatar as "the overall box office leader".

Just spitballing, but it does seem kinda silly not to push for the free PR due to trying to protect Avatar since the MCU is so much bigger