I used the term in a broader meaning. And ad hominem usually means "dont listen to him, he is wrong because he is who he is". You just said that Gamstat is unreliable and that is it. Why is it that Gamstat is unreliable?
No, an ad hominem means "attack against the person". Me calling you stupid is an ad hominem. Me saying a website, in this case Gamstat is unreliable, is not an ad hominem.
Gamstat numbers are unreliable because it takes its data from trophies/achievements. Ubisoft gave away AC3 on numerous occasions throughout its life time and those stats count as a sale, which it wasn't. The people who got it through Ubisoft + would count as well, but it doesn't mean they purchased the game. Therefore, any numbers coming from Gamstat don't paint the correct picture.
Your whole argument is flawed from the beginning. You're assuming just because 25% of people tried the multiplayer at launch, doesn't mean that in consecutive years that numbers would stay still or even increase just because they finished the game. The large majority of people bought AC games for their single player and then they moved on, that's a fact. Sure, plenty of people played it, but there is no way any of your numbers are right without any hard data. A lot of people could've just downloaded, tried it and moved on. Some may have not tried it at all. There's just no way to know.
Then show me when Ubisoft announced sales number for Valhalla.
https://screenrant.com/ac-valhalla-sales-highest-earning-assassins-creed-billion/