The pandemic might be an excuse, but no worse than thinking a different game, developed by a different team, in a completely different country at a different time is somehow comparable in how hard they got hit by a pandemic.
The argument is that SL is terrible partially on account of it's theme, but mostly because the delays have caused players to be left adriftm not knowing how long is left of the expansion, how long until they get more stuff, and of course general burnout being a problem when you are waiting so long.
Imagine for instance if we got everything we have now, but instead of waiting for 9.1.5, we would already be in 9.2. We would have had all the changes already, we would likely already know what 9.3 would be, and the forums would be getting excited for 10.0.
None of that would really change the expansion specifically, but it would have made a world of difference when you play. Instead of getting burnt out on Nathria players would have been more reinvigorated by Sanctum earlier, and instead of complaining about grind we would have likely already moved past that.
The pandemic has impacted WoW massively, and excuse or not it is true.