This is a bit of a myth, especially when it comes to fantasy. More like average players in good systems get shown to be average players in an average system at the Warriors and the Warriors sure love to sign an average player.
Good players still score well in fantasy:
Addin Fonua-Blake - averaged 50 in his last season with Manly, averaged 51 in his first season with the Warriors
Tohu Harris - averaged 45 starting in his last season with the Storm, averaged 50 in his first season with the Warriors
Euan Aitken - averaged 41 his last season at the Dragons, averaged 43 (boosted by second half of the season at EDG) in his first season at the Warriors, then averaged 48 last year
Josh Curran - averaged 52 in his break out season (2021) at the Warriors after joining from the Roosters in 2019
I'm sure there are as many examples of players coming in and playing/scoring well as there are of players coming in and turning into duds.
Warriors main problem is always going to be the appeal of getting big players over to them outside of Kiwi internationals, then those guys are often not in a position of extreme need. Have to make do with often secondary and tertiary options.