This is absolutely it. I've worked on multiple projects simultaneously where I wrote great tests on one project and few to no tests on the other. It wasn't the developer that changed, it was that one codebase was designed with testing in mind, while the other was not and was therefore extremely cumbersome to write tests for. That also meant that in the second codebase whole UI flows had no tests, so if I made one tweak to the UI flow I wasn't going to spend a week figuring out how to test it and everything around it.
