notaverygoodname - I had a conversation with Mr. Alessi last summer about this very thing, about how the ovalform Bb tenorhorn you can buy today isn't all that different from the instrument available in the early 1900s for military bands when the symphony was written, and how it's really a very different instrument than the british baritone horn. Suffice it to say, he was not particularly receptive!
I suspect that while a yamaha neo baritone is fairly easy to get at Dillon music just outside of New York City, it'd be an item they reguarly stocked, a Bb ovalform tenorhorn would probably not be. I do applaud his use of an instrument that is smaller than euphonium for the part as it's more difficult to play that way and I do think it sounds better.
Nevertheless, I do think it sounds spectacular played by him on this instrument. I'd love to hear him do it on a Miraphone 47 or similar.