AMD did things the "right" way, not because they understood the security implication, but because it was very hard to achieve such high level of speculation, and I think the performance gains didn't justify the effort for them.
Happy to be proven wrong however.
A) they are vulnerable to two out of the three attacks, which indicates they did not in fact consider speculative execution a danger
B) it is hard to believe that if they researched the topic at AMD, they wouldn't find this vulnerability in Intel processors a long time ago
It’s this protection that means that AMD processors are not subject to meltdown.
All CPU manufacturers appear to have been caught on the hop by Spectre (branch prediction history side channels).
I think it's worth noting that it's entirely possible, that if you are a CPU execution pipeline designer that you think about memory loads/stores, L1 cache, branch prediction and speculative execution, and it occurs to you that the cache gets polluted by spec exec, and the branches can be security checks.
But the solution is simple. If the branch is important, wait for it. (Load it into L1 cache before the branch - use a memory barrier.)
The fact that this is not in any ISA docs is a likely pointer toward the possibility that it in fact hadn't occurred to them.
These attacks are the same as any new invention. Easy to see once you've grasped the concept.