I’m interpreting the comment above to mean Strava was/is a user of Mapbox and bought Fatmap in an effort to wean themselves off of Mapbox and use “inhouse” (Fatmap) mapping tech.

I took a superficial glance at what the site is loading and iirc there were plenty of the usual suspects in there. And it would be silly to do it any other way, because their product is what they do with maps, not winning some NIH purity award.