I found a "© Mapbox, © OpenStreetMap contributors" in the bottom left when I clicked on an example (https://felt.com/map/Community-Solar-Draft-Proposal-U8dl9A6Z...)
There are plenty [1] of sites & apps (some quite large) that don't correctly attribute OSM data according to the legal requirements [2].
[1]: https://wiki.openstreetmap.org/wiki/Lacking_proper_attributi... [2]: https://www.openstreetmap.org/copyright
I have heard some folks throw shade on MapBox, but it appears to be a pretty good system, from what I have seen.