That would make sense, at least. Every "professional scuba diver tries the Apple Watch Ultra" story I've heard includes a complaint about the lack of air tank support. And yeah, ANT+ would really help with anyone who needs extra sensors for cycling or running.
Apple doesn't need to compete with the best dive sensors or fitness watches, but if it could completely replace an entry-level version of those devices while delivering a much better smartwatch experience, that'd be a victory in my book.