The team did a better job limiting the situations that the goalies were weak on, that's not the same as the goalies being improved. I think Andersen made some (but not enough) improvement on losing focus and letting in weak goals, but Gibson looked every bit as bad at breakaways and crossing the crease as he did the previous year after injuring his groin.
Improved statistics because of a tighter team game =/= goaltender improvement. The reason that matters is that RC will not be playing the same defensive system as last year, so the only constant will be goaltender play, and if the new system isn't as good at limiting opportunities that take advantage of Gibson's weak points, his numbers will suffer unless his actual play improves.
A "tighter team game" doesn't preclude goaltender improvement, either. Attributing moist of the significant improvement in Sv% and GAA to the skaters in front of them is unreasonable in my view. Sure, team defense helped them out a lot this past season, but it didn't do their job for them.
Also, the fact that the Jennings Trophy is a team accomplishment does not diminish the fact that the goalies are a big part of that accomplishment. After all, the goalies are awarded the trophy, not the team.
Our goalies aren't perfect, but by the measurable evidence that I'm looking at, there is no question that they improved last season.