Of course Tesla tests things itself with its own drivers. What they can't do is replicate every single one of the unusual corner cases that people encounter in the real world.
Taking out of context by cutting off your sentence, but pretty much any drug has a listed side effect like this.
From the linked tweet, there's no way to evaluate how unusual the failure case was. And of course we don't know how likely it was to have been a pedal misapplication. We do know that that's not uncommon, though.
The original comment I replied to assumed that there was no internal testing, and you're assuming there was very little internal testing. Both of you are jumping to that conclusion with very little support.