"The plan is just a version of the negative income tax. Milton Freedom first proposed it...and as a student 40 years ago I thought it was a pretty good idea. And it turns out I wasn't alone in that judgement...In 1968 a 1000 economists endorsed a negative income tax along these lines. What Andrew Yang is proposing is a version of what these 1000 economists endorsed back in 1968. Could 1000 economists all be wrong? Laughs Well yes they could. But I don't think in this case they are. My own view is that a universal income financed by an efficient tax, something like a value added tax, might well be worth considering. And I'm one of the signatories...of a plan to do something similar with a carbon tax."
As in, the $1k/month thing is too big, and he thinks we should consider doing something smaller. How much smaller? We don't know for sure, but I doubt economists would support a carbon tax that would approach the level of $1,000/mo for every adult in the US. The primary purpose is to internalize the externalities of pollution, not to generate revenue.
The fact that an economist thinks a concept is worth considering is a far cry from saying they are endorsing a specific instance of one of those things. It's one thing to say he endorsed the concept. It's another thing to say he endorsed the plan. He didn't.
It does not read like an endorsement — just an assessment that Yang's plan is better than a plan he describes as unworkable (Warren's).
In the Youtube, he says: "My own view is that a universal income financed by an efficient tax, something like a value added tax, might well be worth considering" — hardly a full-throated endorsement.
Has he said anything else that is more positive? Or more specific with regard to the amount of UBI that he thinks would be appropriate/efficient? I wouldn't call either of these things an endorsement.