Never implied that anyways. Official merely ensures that the amount of trusted parties can be minimized.
Bubblewrap is not insecure.
Bubblewrap, when properly applied is indeed excellent; perhaps the best utility to sandbox applications on Linux. I’m thankful that flatpaks makes use of bubblewrap, namespaces and seccomp to offer relatively safe/secure apps/binaries, I’m unaware of any other ‘(universal) package manager’ within the Linux-space that offers similar feats in that regard. Unfortunately, Chromium-based browsers just happen to have an even stronger sandbox -if properly configured- than flatpaks are currently capable of.