It's true. I've read one side channel attack paper that admitted towards the end that the researchers didn't actually even have access to AMD hardware to test with, but thought it should work in principle.

Fact is Intel takes the brunt of these attacks because their HW is more available especially in semi-standardised environments like universities. Also, Intel fund researchers (this paper was part funded by Intel), and their technical documentation is better than AMDs, at least in my experience. That makes it easier to understand the CPU internals, which is a big part of these papers.