The article also says that the original P5 shipped 3.2m as of Dec 2019. That puts total sales for both versions of P5 at over 4.6m.
I looked up DQ 11 and it has shipped over 5.5m copies worldwide as of Dec 2019. That's still more than Persona 5, but I'm impressed at any JRPG that can come close. For a long time Final Fantasy and Dragon Quest were considered the 2 big ones, but now Persona is turning into a close third.
It's a bit unfair to look at P5's numbers like that considering most of the people who got Royal are likely double-dipping.