The first tape out on TSMC's 5nm was back at the beginning of 2019. Apple's new phones are using it, I believe. It is possible that NVIDIA have been stockpiling for a while now.
That would help, but in all honesty, they could end the problem now if they wanted to.
1.) AMD/Nvidia could require AIB's to have no large volume sales other than to end retailers
2.) As a condition of sale, a contract is signed with the retailer to achieve positive identification of all customers, and have a "max 3 units per customer" type of policy.
Sure, miners could do the Ephedrine Smurf thing meth cookers used to do, but it likely would seriously cut into their profits and not be very effective. They are only cornering the supply today because they either outright sign mass volume deals directly with the AIB's or they are buying up supply with bots. Have a positive customer identification scheme in place and you end the latter. The volume sales is a matter of choice. They either make it or they don't, and thus far they have.
In the end - however - the supply restriction is on the fab side, not on the video card side, so if they do this, they will likely just find themselves competing with others, maybe ASIC manufacturers for the same fab capacity at TSMC and Samsung and we may find us back here again with short supply as AMD and Nvidia can't get fab supply.
It takes 5-10 years and billions of dollars to get a cutting edge fab online, and it is really technically challenging (just look at what happened with Intel's 10nm. If even Intel can screw it up, it is a real challenge) Globalfoundries and others have just given up on cutting edge fab tech, because it is too difficult.
Unless something changes to reduce demand for mining (crypto crash?) and other purposes, I think this problem is unfortunately here for the long haul.
The tech economy has gotten used to readily available latest node fab supply, but the explosion of mobile and IoT and crypto mining, as well as an increased demand for PC gaming parts in the last 5 -10 years has put a huge strain on this supply, at the same time as it is more difficult than ever to actually make these nodes work.
The "Creating Helpful Incentives to Produce Semiconductors for America Act (CHIPS)" bill might be helpful in this regard if it passes, but it doesn't seem like it is enough given the extreme expense and difficulty to develop fabs, and even if it passes, results from it are 10 years out.