I really hope Garmin doesn't get bullied by Apple, and continues to exist. Between Google and Apple, Garmin is a notable competitor, I'd argue Garmin products are superior too.
Needs are required for life. You don't need sleep tracking, you want it.
GP didn't make it explicit but you can probably assume they were implying one of these contexts, rather than that they would die without that feature.
The smart watches with blood oxygen saturation sensors can be useful for detecting sleep apnea. But that only applies to people with the condition.
Why do you think you need sleep tracking? Is the data actionable?
If we want to improve sleep quality then it's no big mystery what to do. Consume less alcohol and caffeine. Exercise more. Don't be obese. Don't eat a large, protein-rich meal late in the day. Go to sleep earlier. Keep the bedroom cool, dark, and quiet. We all know what we ought to do, we just don't reliably do those things. Better sleep tracking won't fix a lack of discipline or give us more hours in the day.
Also, recently I bought Polar HR10 heartrate monitor and was unpleasantly surprised by the fact their Android app is also "signup required", for practical purposes. I mean, it does let you use it offline, but I never found a way to actually export my data (I suppose it should be doable in their web-app, though). And I didn't find any alternative HR-logging app for Android that would allow me to do that, so that I could use the product I bought the way I want. Well, nobody promised me I will be able to, but I still feel kinda robbed.
iOS app limitations though, I agree. Fits their pattern of behavior.
Overall the list could go on for days, I think you nailed it on the head that it would fit their pattern of behaviour. And they already seem to be following that pattern with their current line of watches and what they can do versus what a non-apple watch can do interaction wise with their phones.