Luigi's Mansion 3 and Splatoon 2 hitting 10 million wasn't outlandish, especially not the former. By the time Luigi was on the way, Switch was a massively successful system and it was well established that first party games were selling extremely well. Given Luigi's Mansion 2 exceeded 6 million on 3DS, 10 million for the sequel on Switch makes a lot of sense. Splatoon 2 was naturally more difficult to call, given Switch itself was an unknown quantity.
But it shouldn't be assumed that highly-rated Nintendo IP all go on to shift enormous numbers on Switch. Look at Mario Maker 2 - a major EPD title which sold strongly but didn't make it to 10 million. Fire Emblem hit the 3 million mark, representing a great success for the series but also showing there wasn't that much room for the franchise to grow beyond the success of Awakening, and that is a critically acclaimed game with a similar critical reception to Metroid.
Consider that the most successful Nintendo titles on Switch all have a level of appeal that is effectively "all ages". That isn't the case with Metroid Dread (nor Fire Emblem) which will skew towards a slightly older audience. It's also the case that no non-Mario 2D title has exceeded 5 million units on a Nintendo system since DKC Returns, with the exception of Link's Awakening 2019. Another factor is that Metroid is building from a lower level of base popularity than other series - it has at times struggled to crack the 1 million mark, and has only twice broken the 2 million mark. Its popularity peaked 2 decades ago. There's a lot more going on, and a lot more to take into account, than "Nintendo IP are all wildly successful" on Switch, when thinking about how a title gets to 10 million sales. Splatoon and Luigi tripled and doubled their previously totals, respectively, but think about what double or triple growth would represent for Metroid Dread.
Nobody is underselling Dread when they say 3 to 5 million would be an impressive sales range. Let's not forget that when Metroid Dread does go on to sell 3 to 5 million units, it will have sold six to ten times as much as Samus Returns and Other M did, and 3 to 5 times as much as Metroid Prime 2. It would mean Dread sells at least twice as much as Super Metroid, Metroid Prime 3, and Metroid Fusion, or potentially 3 or 4 times as much.
Is it impossible that Dread triples the sales of the original Prime, and sells 8 to 10 million? No. But let's not pretend that scenario is anything other than a long shot.