This is the key question. Blizzard allows Guild Masters to control their own guilds. That comes with some responsibilities such as setting up permissions, controlling guild ranks, and controlling invites.
If a player took items they had permission to take - Blizzard will not intervene.
The exception is if that access was granted because of a compromise or scam. That one is sort of hard to prove, but that is the rare circumstance they assist with this for.
It is really important that the Guild Master learn about the tools for managing a guild and use them.
They put this together in 2018
Here is the official article on the policy against most Guild Bank restorations. It also gives some tips and help on setting up permissions to protect a guild bank.
Customer Support does not assist with reclaiming gold or items when removed by someone using Guild Bank access they were granted through their given Rank.