It isn't me or the ex-Square guy, it's the reality: yes, there are dozens of multiplat every month, most top seller games every month are multiplat and most of the total game sales in gaming are from multiplat games.
There are games like GTA, Minecraft, Cyberpunk, Dying Light 2, Jedi Fallen Order, Lego, Assassins Creed, Far Cry, The Division 2, Rainbow 6 Siege, Destiny, Mortal Kombat, The Witcher III, and a very long etc. that you'll see frequently in best selling rankings.
And yes, if you release your game in more platforms you have a bigger user reach, so you'll sell more total units specially if the other platform holder supports the marketing of your game. It's basic maths.
This is the reason of why most 3rd parties started to move to multiplatform as costs kept rising, and in recent times we even saw the three console makers publish games outside their own platforms. Companies want to grow their revenue, something it's mandatory in AAA because every gen they get more and more expensive to make and market.
