There seems to be some confusion here. If someone is claiming that they expect AMD to have up to 30% "market share" in a particular quarter, they're obviously referring to sales during that quarter, not to the existing install-base suddenly seeing massive changes over the course of a few months, as that simply doesn't happen. It's certainly possible that AMD could see substantial gains relative to Intel for a particular quarter, particularly given Intel's current production issues.
Also, it depends on what market, exactly, we are referring to. If we are talking strictly about the sale of boxed processors at retail, then AMD seems to be doing quite well right now. Going by Amazon's "Best Sellers" list for boxed CPUs, we can see that 6 out of the top 10 best selling processors at their site are from AMD at the moment. And 2 of the 4 Intel processors in that top 10 are pre-orders for the 9000-series chips that were just announced, and are not even shipping yet...
https://www.amazon.com/gp/bestsellers/pc/229189/