The Knicks' only exceptionally good 3P% shooter last year that isn't currently here is Steve Novak. Jason Kidd is gone too, but he shot 35% career (shot 35% for the Knicks last year).
Everyone else that shot 3s is still here, and everyone who is still here shot 3s +/- 2% of their career 3P%. Statistically speaking, there's no argument for decreased ability to shoot/make 3s, unless you want to say that all of these players (who essentially only shot their career averages from 3 last year) collectively had an outlier.
The Knicks 3-point barrage was via sheer number attempted, and by design - not by exceptional percentage. It's not because any one player is an excellent shooter...it's because practically the entire team can/will shoot 3s at the very least at league average.