He also played 4+ more minutes per game, on the 2nd highest scoring team in the NHL, and had 17 PP points compared to only 5 for Garland. An extra 20 points isn't all that impressive, in that context.
So what you are saying is Nylander's value should be decreased because he's good enough to be a regular fixture on one of the top scoring teams' powerplay? A team with plenty of high end offensive talent for him to beat out for that spot.
In the year that's being talked about Carl Soderberg had more powerplay production that Garland, on the same "his value should be higher than his production indicates because he plays for" Arizona Coyotes. Is Soderberg also equal value to Nylander then?
There's lots to like about Garland. He's improved every year, and his significantly less NHL experience vs Nylander can be viewed as both an asset and a detriment. An asset because his lower production years relative to Nylander are potentially just him adapting to the league. A detriment because the greatest determinant of value to NHL GMs is what you've ACTUALLY done. Garland might legitimately turn out to be as good as Nylander, maybe better, but the discussion for him having similar value is for ~2 years from now, when/if he's done it consistently for an extended period of time as Nylander has. Since his contract is expiring, that also happens to be a time when he has a new contract and isn't making "9 times" less than Nylander.
No one thinks Liljegren is worth much. Leafs fans can go on about how he's been the marlies #1 D, has become a really well rounded D, etc. That doesn't mean any NHL GM is gonna pay a top 4 dman price for him, because he hasn't done that yet. They still won't even if he gets called up and at some point has a steak of 35 games where he plays like a top 4 guy. The leafs best bet is to give him a chance to play and hope he can prove he can be that. Arizona's best bet is to re-sign Garland and hope he continues to play the way he has the last 35 games.
35 games producing like another player has for half a decade isn't enough to make them the same value, even if they are on a cheap deal... that also happens to be expiring.