I agree. Last year the comparison had to be vs 2017 switch, and this year vs it's 2018. However who would think that the problem with shortages will be so big that holiday season for the system would be what 2 normal months with normal stock would have been ... Because of that last year PS5 instead of doing 15 or 16M (close to 2018 switch) it did just 13M (what switch did in 2017). This year I could again do 2018 comparison but it would be repeat comparison of the last year and it may annoyed some people. Anyway there is a chance for PS5 to make close numbers to 19M, since the stock issue have to be resolved by large extend by the end of the year.
You do have a point, that sales were notably limited due to lack of stock. However, even with the stock issues figured out I think people are overestimating it's sales numbers. Currently, the console sells out quite a bit due to the limited stock phenomenon (where people will buy something that they may have waited for because the stock is currently limited). Not saying the demand is "artificial" or anything like that. It's definitely there, and sales would absolutely be higher without stock issues. However, a 19M+ year for it's second full year on the market is a VERY lofty goal for any console. 2019 is technically Switch's third year on the market, which is partially why it hit that goal (among just having a stellar year software wise and, of course, Pokemon). Sony is looking to have a great year software wise as well, but unlike Switch all three of it's best sellers will be on another console which people own (PS4). In addition, it took Sony until 2017 to have a 19M+ year for the PS4 when it's competitor practically shot themselves in the foot. It would be difficult for the PS5 to do that in half the time when it's competitor is doing substantially better image wise this generation.
Anything can happen, and if Sony sells 19M+ consoles in 2022 then that is freaking awesome. I just don't believe they will or that they would even if the stock were figured out. Just my two cents.