He's not claiming he invented the technique
From your own article: "He betrayed me," he said in his strong German accent. "I invented 'wet on wet.' I trained him and he is copying me -- what bothers me is not just that he betrayed me, but that he thinks he can do it better."
Your NYT quote from Ross describes one interview with him and the NYT. Per Wiki:
Ross dedicated the first episode of the second season of The Joy of Painting to Bill Alexander, explaining that "years ago, Bill taught me this fantastic [wet-on-wet] technique, and I feel as though he gave me a precious gift, and I'd like to share that gift with you [the viewer]"
You can see the dedication in the first minute of the episode
Note that Ross doesn't falsely claim to have invented a centuries old painting technique while pouting, he just said "Yeah, someone taught me this but he's a competitor now so I'd rather not discuss." That's classier than outright lying about having invented something.