YES! The key was the commands capability request:
await _twitchBot.SendCommand("CAP REQ :twitch.tv/commands");
After that, the commands work as expected.
Even the /help and .help do show the response as NOTICE messages.
Also, I find it makes no difference experimentally if the CAP REQ happens before or after the JOIN. That kind of makes sense since those requests are not channel specific?
Thank you for your help!!!