Well why isn't it factored in? Minny fans are so quick to discount Robertson success since he plays on a "stacked" line with a 37 year old Pavelski but Kaprizov gets a pass playing with Fiala on the PP where 1/4 of his points come from?
I don't think anyone is saying the line Robertson plays on is the most stacked in the league, it's just a comparison between the line that Kaprizov plays on and the line that Robertson plays on. It's not that deep it's just the counter to the 5 on 5 argument. Yes, Robertson has produced more points even strength. Wild fans are just responding saying that it's true that Robertson has produced more 5 on 5, but that, in a vacuum, doesn't take into account the differences in the strength of the lines that the two players play on during even strength minutes.
Again, that doesn't mean that Robertson is bad. It's just a reason why Kaprizov doesn't produce quite as many points even strength. Kaprizov has less help, both in generating chances for himself in scoring areas and in his linemates finishing chances he generates for them. Wild fans are saying it's incredible how much Kaprizov has been able to produce given the quality of his linemates at even strength. Again, none of this means Robertson is a bad player.
It would be interesting to see a stat like a "high danger chances created per game or per 60" and see how the two measured out. Again I want to stress, none of this is saying that Robertson is bad.
To further make a point of why this matters, you or another Stars poster acknowledged that the Stars haven't had a lot of NHL talent available outside the Robertson lined and "no one not even McDavid" could produce with the Stars 4th line or some of the talent they have.
This is an acknowledgement that the level of players that you play with does matter. If it didn't matter, than how could no one produce with them? If the level of linemates does matter or does impact the point production of a player...than how is discussing the difference between the quality of linemates between Kaprizov and Robertson when they play at ES not a fair and relevant discussion point?
The only point that Wild fans are making is that Robertson would produce less at even strength than he does now if he had worse linemates and Kaprizov would produce more at even strength if he had better linemates. Robertson has worse linemates than Kaprizov which means that Kaprizov would produce more at ES than he does now with Robertson's linemates and vice versa for Robinson with Kaprizov's linemates.
This does not change the caliber of player that either player is. The point is that you know how good Robertson is and him producing less with worse linemates doesn't make him a worse player... he just has less support and thus less opportunities to produce points or has to do more himself in order to do so. Point production is one measure of skill and value to a team, but we can dig deeper and add more context to numbers to make them more informative and truly discuss what they mean and don't mean.