Yes that's true, but not a problem for TCP since the attacker had to know the outgoing port from the ephemeral port range your PC or router picked, and then had

Yes that's true, but not a problem for TCP since the attacker had to know the outgoing port from the ephemeral port range your PC or router picked, and then had to take over the running TCP session by additionally knowing the connections state (sequence number etc). For UDP you indeed have higher chances of talking to that device in an unsolicited manner, and this has been exploited by games and later Skype in the past. So yes, indeed more insecure, I stand corrected. I'm still more worried for actual exploits or security holes even in modern devices that can do proper firewalling the ISPs provide due to incompetence or laziness. Like the TR069 port being open on many devices two years ago.