It doesn't go up. It's going to drop in that scenario because AMD stock price will drop. You don't need to wait for earnings to come out in 3 months to have the PE ratio drop. 🤦♂️
It's a hypothetical analysis starting from the basis of AI revenue going to 0. The instantaneous P/E goes up and then the market reacts. The end result is the same and if you want to start the analysis after the expected market adjustments or at the moment of hypothetical revenue drop doesn't make a difference and is simply a game of semantics.
If you are talking speculation, then you should look at forward P/E but if you want to strictly look at current P/E, then AMD is priced to roughly double their current earnings compared in relation to NV's P/E ratio (trailing 12 month P/E which is not the best metric for a growth stock but I'm being generous to your argument). If even half of AI goes away, NV's revenue gets cut almost in half and their earnings drop even more because NV's margins would go down significantly as well. AMD's revenue drops by roughly 10% but margins actually improve because their DCAI products have significantly less margin than their company average so the effect on earnings from loss of DCAI revenue is actually mitigated by increased margins even without growth in other segments.
So, all of a suddent the ~2X difference in P/E between NV and AMD becomes roughly a wash with the change in earnings. Then compare the growth outlook with a stagnant AI market and NV has no growth to look forward to but AI is now only 10% of AMD's revenue and is still experiencing growth in their gaming, client, and DC-CPU segments to drive future revenue growth with increased earnings leading to a lower forward P/E than NV. So, in the case of an AI bubble pop, unless NV can find additional revenue in other segments with extremely high margins, they face a much harsher drop in market value than AMD once the dust settles.