The word "years" suggests they aren't just talking about the new arrhythmia feature.

Yeah, Apple aren't known for being entirely honest about the scope of their innovations. Suffice the say, that's Apple parlance for "a handful of times the past year."

It's likely they saved lives with previous models too. I work at Fitbit (don't speak for Fitbit) and we often see stories of people who say their Fitbit saved their life because it helped them notice some heart condition or other.

Nitpick: you can “turn them on” without an iPhone.

Feel free to explain how, because all the research I've done suggests that an Apple Watch is completely unusable without an iPhone to set it up.

You can turn it on, but it’s unusable (hence this being a nitpick).