Merging into a single branch isn’t the definition that people like ThoughtWorks seem to use. They just talk about integrating into a shared respository, not specifically a shared branch.
The CI system then does the work for you to test that on top of master. You’re talking about developers doing things that the CI system is there to do.
CI only seems to make sense to me when talking about a single branch ("master", "trunk") into which all changes are frequently merged. FWIW, also Wikipedia describes it as "merging all developer working copies to a shared mainline", the latter being a branch, not repo.
Otherwise, you still might test indvidual feature branches on top of master, but that wouldn't prevent any conflicts between several of the pending branches. One could think of a system that continuously tests all possible combinations of all pending feature branches to spot any conflicts before actually integrating them. Not sure whether such thing exists, though, or whether it'd be very practical.
The CI system then does the work for you to test that on top of master. You’re talking about developers doing things that the CI system is there to do.