There's a difference between binary is/isn't fragmented and degree of fragmentation. The browser market is obviously not very fragmented with Chrome taking the majority of the market share, and top 3 choices together account for almost 90% of total market.
However you like/dislike any of the popular choices, the point is that other players in the ecosystem can mostly target the top choices and be reasonably sure that they cover the vast majority of user bases, so that upper layers could flourish. Except for Edge being the default on Windows, sane companies wouldn't care about minor players like Opera/Brave/Arc.
(You could also argue the degree of open source in these cases but that would be a different issue.)
However you like/dislike any of the popular choices, the point is that other players in the ecosystem can mostly target the top choices and be reasonably sure that they cover the vast majority of user bases, so that upper layers could flourish. Except for Edge being the default on Windows, sane companies wouldn't care about minor players like Opera/Brave/Arc.
(You could also argue the degree of open source in these cases but that would be a different issue.)