Reply To: At 14:40 ish of the video, `override data block`

[Jonathan Lampel]

Ah, it looks like there’s a new option in 2.9 to set custom properties as  explicitly overridable and it’s off by default.’

In the original file, turn this on in pose mode in the hand’s bone properties:

Edit: I’ve updated the source files to include a 2.91 version of the model that includes this change.