IMO the biggest reason why the Apple Watch is oftentimes interpreted as an "unserious" exercise smartwatch is actually quite simple: The display & lack of physical buttons makes it difficult to interface with in the variety of conditions that outdoor activity enthusiasts often find themselves in. If its bright out, the mps displays on many Garmins will outperform OLED. If its raining; good luck using a touchscreen. If you're wearing gloves; ditto. If you've just ran a marathon, you're dying, your vision is blurry, you're sweaty and collapsing, that "swipe over a screen then click the end workout button" workflow is the end of the world; it wasn't designed by someone who has ever been in that situation, its designed for and by people who take their nice little walks to the cute little grocery story.

Battery is another less major factor: Even the AW Ultra 2 struggles to make it through a full marathon run (~70-90% battery usage IME) and that's not an uncommon-enough situation for users of the quote"ULTRA"endquote to be an invalid criticism.

Nothing else matters. Your comment continues into talking about sleep tracking and recovery scores and strain scores and third party apps and literally none of that matters. That's silicon valley brain stuff that many customers don't care about. The Apple Watch is, to some people, a Bugatti without a steering wheel; it gets a lot of the basics wrong.

One note though: Many Garmin users would also say that Garmin is, sadly, also losing track of what their core userbase wants, as the experience has become more buggy and less focused over the years. I'm not asserting that Garmin is king and Apple are idiots; Garmin just has momentum and is generally great at the things its users care about.