As already mentioned by others, weight isn't as great a concern as it would be on road race climbing stages. Deep frame and wheel sections, in addition to drinking systems ensure that tri bikes aren't too threatening to the 6.8kg limit. But, yes, to the best of my knowledge there isn't a weight limit for tri bikes. The 3:1 frame element ration also doesn't apply to tri bikes. Hence the Tri-Shiv vs TT-Shiv from Specialized.

However, it's the saddle set back and bar extension measurements that really seperate a UCI position from tri positions. In this instance, Gordon could probably comply with the UCI TT regulations, as he really isn't taking advantage of the forward rotation that tri's lack of regulation allows.

The consequence to UCI legal TT positions is that you'll see saddles further aft than the same rider might use for tri, with bars that are also set further aft, making it more difficult for some to achieve a low and flat back.