> I see your point, but it feels more like the difference between 99% fat free and 100% fat free.

Then why not have the headline say "99% open source"? IMO, either it doesn't matter and you can just say that it's almost all FOSS, or it does matter and you really shouldn't be lying about it.

> Technically measurable, but irrelevant in practice when the alternatives are all basically closed source.

The alternatives are the pinetime, watchy, and bangle.js, which AFAIK are also ~95% FOSS. I guess Apple and Google also offer smart watches, but I'd argue that those are so different in terms of features that they're not really direct competitors.