That's just not true. Having access to the microcode isn't going to help you here, there won't be a line of code you can point to and say 'this is a bug!' Rather, it's the assumptions that all chip-makers have chosen that were wrong, and those choices have been out in the open and known about for all this time. No-one was demanding that speculative execution should be hardened against timing attacks because no-one had discovered that timing attacks were even possible.