I just read it as easy language. Guanches is a term few people understand unless they've been at least a visitor, whereas Canary Islands is much more widely known.

But apparently there's more to it: in the language the term comes from, it specifically means person from Tenerife. So yes, we might consider it respectful to not refer to people on ancient El Hierro as Guanche. You might want to take a look at how es.wikipedia is linked from en.wikipedia. As a tourist to Tenerife, I didn't know about the nuance myself, before today's wiki dive.