I agree with the core of your post.
Although at a certain points you are comparing apples and oranges. I’d judge the battery life of both units at the settings which you would encounter in real life. A head unit that I can’t read is not that useful.
In a sense the great-to-insane battery life of their x40-series headunits exacerbates the issue. Going from 15 to 12 hours is one thing. [The 840 solar had 25–40 hours IRL](Garmin Edge 840 Cycling GPS In-Depth Review | DC Rainmaker), and 12 hours is less than half! Personally, if Garmin had cracked 18–20 hours, I reckon it’d be more than good enough. The 840 was a no-brainer to recommend whereas the x50-series head units come with asterisks.
That’d help mitigate the issue, but IMHO the issue still remains. We rightfully criticized Wahoo for its overly optimistic battery life estimates for the v3 Roam and Bolt, which you can only achieve if you use settings that degrade user experience.