I definitely think this Joy-Con update needs further investigation. I have one problematic left Joy-Con that before the update you could barely be a metre away from the Switch before it would start malfunctioning, and now seems to work if I stand ~4 metres away and holding the thing behind my back (which would always make it disconnect before the update regardless of distance away).
I wish I'd have tested this directly before and directly after the update, because that would have made it more apparent whether this is just me overestimating the problem before or not, but I'm about 90% sure there's something there and this update somehow improves connection.
Edit: And just to be 100% clear, this is a launch Joy-Con that I have never sent to Nintendo for repair or done any modification with myself.