From that, it sounds like ECI have fixed their side of things, and then put the update through their own set of function & system tests - almost certainly under close supervision by BT.
Now it has been released to BT, they look to be putting it through their acceptance tests, to try to break it. The "extensive and formal" wording suggests that testing is being redone in its entirety, not just targeted regression tests. "Major new functionality" seems a bit of a misnomer here; "fix to old broken functionality" would be better, but the loss of trust means it is being tested as though it is "major new" stuff.
It'll be interesting to see if BT set the initial playing field to be "just" G.INP activation, or if they will try to activate the XdB changes in the same package.