I base my statement that sending a UDP packet down a connected socket is perfectly possible on the description of the sendto() function at this link:
https://linux.die.net/man/2/sendto
…where it explicitly tells you how to do it. That and the printf()s showing it happening. You aren’t the first people to tell me that you can’t send UDP packets on a connected socket though: can anyone point me at a reference for this rule?