I did some googling, and apparently this is not true. 
In fact, git bisect does apparently search feature branches, not just merge commits.
To have it ignore commits “internal” to feature branches, you apparently need to jump through some additional hoops: https://blog.quantic.edu/2015/02/03/git-bisect-debugging-with-feature-branches/ and How do you get git bisect to ignore merged branches? - Stack Overflow … though recently this apparently became easier with git bisect --first-parent