It's really not even leftwing. It's the purest form of capitalism, but it would move us closer to the leftwing utopia than any idea the Yangs of the world have ever proposed.
To be clear, Andrew Yang is a georgist. He advocates for a LVT and his whole platform is modernized Georgism. Henry George and Thomas Paine absolutely did advocate for UBI (I think Paine originated it in Agrarian Justice)
Henry George definitely did not advocate for UBI in the absence of already solving the problem of land rent capture. If you do solve that problem, then there are a million other solutions you can stack on top of it. If you don't solve that problem, then none of those other solutions mean anything.
George actually specifically "disproves" UBI as an actual solution (in lieu of LVT).
Yang has said he "likes" LVT, but has advocated for a million other taxes and solutions which, without LVT, won't work, and with LVT are probably just unnecessary drag on an economy.
It's way, way too generous to say his whole platform is modernized Georgism.
George is fundamentally a capitalist while Yang is fundamentally a socialist. The fact Yang is down to tax land does not really make him a believer in the overall mechanism and argument behind it, and his policy proposals prove that he is not.