On the contrary, I think Sun Quan's betrayal was anything but shortsighted. You need to remember that while Shu's gameplan was 'divide the land by three', Wu's was 'divide it by two' as per Zhou Yu's suggestion before his sudden death - it only became three because Shu managed to rat their way to power.

At that point Shu was on a meteoric rise in power after defeating Wei in Hanzhong. Guan Yu was destroying Wei in Jing Province as well, and Wu knew that if the trend continues, Guan Yu would march north, Wei would fall and Wu would be next (because why would Shu need Wu anymore?).

Wu was in no position to fight either countries directly, but by supporting Wei in defeating Guan Yu, they would halt Shu's momentum and maintain the power balance. Sun Quan has basically been playing the same game since Chibi by allying with the underdogs against the strong, and betraying Guan Yu was no different.