It would be a major upset if Xbox sold more than PS globally. In the USA I can see Xbox selling more because of stock allocation, sure. Microsoft doesn't have the worldwide appeal to necessitate an even spread of their stock, so they can load up the USA.
In a hypothetical situation where both companies had limitless supply, Sony would definitely sell more units worldwide, because too many countries dislike Xbox. But reality will have limited supply, so the different distribution between individual regions favors an Xbox win in the USA while PS still wins globally.
I'm having the hardest time following your line of thought here. Are you saying that if Sony manufactures 4.5m units and MS manufactures 5m units, the PS5 would still sell more globally?
Yes, something like that, but rather a scenario of Sony having minimum 5m while Microsoft has more than that. Sony can be expected to sell that amount of stock, but Microsoft cannot.
To elaborate, the PS4 has beaten the XB1 by huge margins in mainland Europe (ratios of 7:1 or higher in individual countries), so there's a high prevalence of "everyone I know has a PS, not an Xbox, so that's the brand I'll stick with unless the new Xbox is a greatly superior product than the PS5." That's not the case for the XSX|S, so if Microsoft shipped a whole 2m to Europe, including the UK, they wouldn't be able to sell it through.
It isn't the case that whatever gets shipped will be sold and that's probably best explained by using Japan as an example, because it works well for both consoles. If Microsoft shipped 300k units to Japan this year, that wouldn't translate directly to sales. If Sony shipped 1m units, that wouldn't sell either despite the double punch of launch sales plus holiday sales. If you asked the question how much each console could sell in Japan in 2020 if supply were limitless, then you'd likely put the absolute ceilings for sell-through at 100k for XSX|S and 700k for the PS5.
Ceilings exist in every country, but the sheer size of the North American and European markets means that sellouts are common for console launches, so it may appear that everything that gets shipped will be sold through. Last generation the PS4 shipped ~4.5m globally and was sold out, so Sony could announce 4.2m sold through by the end of 2013. Microsoft announced shipments of 3m in response, but the XB1 wasn't sold out globally despite the limited European launch allocations. The USA got the lion share of initial Xbox shipments (~2m), so the PS4 and XB1 were neck and neck after the holidays there. But in Europe it was never going to be a contest and that's not going to be different this time either. The XSX|S is a better product than the XB1 was, but demand between the PS5 and XSX|S in mainland Europe heavily favors the PS5.
That's why when you go to 5m+ global shipments for XSX|S, you have to ask where those units are supposed to be sold through. 3m in North America seems feasible because the USA like Xbox, but 2m elsewhere is a tough ask.