AMD's statement is specifically directed to the exploitations in the Google Project Zero research. Don't assume anything beyond that. Just because those methods may not work on AMD CPU's doesn't mean that other attacks on speculative execution on AMD CPU's won't work.
Again, from the researchers:
"As the attack involves currently-undocumented hardware
effects, exploitability of a given software program
may vary among processors. For example, some indirect
branch redirection tests worked on Skylake but not on
Haswell. AMD states that its Ryzen processors have “an
artificial intelligence neural network that learns to predict
what future pathway an application will take based
on past runs” [3, 5], implying even more complex speculative
behavior. As a result, while the stop-gap countermeasures
described in the previous section may help
limit practical exploits in the short term, there is currently
no way to know whether a particular code construction
is, or is not, safe across today’s processors – much less
future designs."