"Indie" doesn't refer to independence from a platform, it's supposed to mean independently published (in other artistic industries, this has become slightly muddled as "indie" is more of an aesthetic these days). The fact that these games are directly supported on one platform over others is a symptom of being indie more than anything, as a large publisher would require support on multiple platforms to ensure maximum market penetration.

Also, proton is incredible these days, so windows lock in isn't as much of a thing anymore.