In the case of Xbox yes, they hide their numbers because compared to Sony and Nintendo the amount of games and consoles they sell are crap. This is the reason of why are they leaving Xbox and moving to other platfroms, specially PC, and also leaving game sales to replace them with a subscription. To see if at least they can shine there.
In the case of Sony they only share sales numbers of a few games from time to time, and they don't need to be the most successful ones. As an example, in their latest PR they included a game from last year: Miles Morales. But they didn't include 2 other games they also released in H2 2020 that are way more successful: TLOU2 and GoT.
Does it mean TLOU2 and GoT tanked? Not at all. It just means that Sony only includes a handful games in the press releases, those that in that moment make sense in PR terms. This time they decided to share sales of the PS5 1st party games, and Demon's Souls isn't one of them.
Pretty likely somewhere soon they will announce the acquisition of Bluepoint, and they mention Demon's Souls sales in that press release. Or maybe they will wait until the game reaches certain milestone, like 3 or 5 million units sold.
The press release included the sales of their 1st party games, not sales of 2nd party or 3rd party ones. Demon's Souls not being there doesn't mean it had bad sales.
To guess they were around 10M was easy, and they have been wrong constantly always. Obviusly from time to time after making a gazillion guesses eventually some of them may end being accurate.
They were niche when the original Demont's Soul was released. But Bloodborne, the Dark Souls games and Sekiro had very good sales and Elder Ring has been the most hyped game of E3. Soulsborne aren't Fortnite, but they have a great sales potential. Bloodborne 2 or their next IP could easily achieve over 5M copies, and maybe even may end reaching 10M, something only a few Sony games achieved.