Definitely not a 'red liner'. That was dumb on Obama's part because it was obvious that he wasn't prepared to act on it. Red lines should be reserved for when you are almost itching for it to be crossed.
I don't understand the strategy here if there is one. Also, I don't think Trump ever signaled that there was a line that Assad shouldn't cross.
If the US is doling punishment for breeches of international law in a general sense, then that's a new development.