I’m rather confident law enforcement will leave no stone unturned here and we’ll learn more as they investigate the shooter’s communications prior to the attack, but I do think there’s a slight bit of confirmation bias on display here. It doesn’t seem unlikely in the least on its face that Robinson’s partner knew about the attack, but I’ve yet to see any actual evidence of it and law enforcement doesn’t seem to be treating the partner as a person of interest right now. It fits the expectation very well for the partner to be involved in some way, so I think there’s some reading between the lines happening that isn’t seemingly supported by what has happened since the arrest.
It wouldn’t surprise me if the shooter showed his partner the bullets as a hobby or art project.