Contrary to the rumor you heard, Kubota makes its own backhoes and front end loaders, even for the little BX tractor. If you look first hand at the Kubota BH's, particularly on the larger tractors, it's apparent they aren't the same as what Woods offers.
As others indicate, if you want a factory Kubota backhoe for a BX, you have to buy the BX25 (or a decent used one - BX22, BX23, etc.). The BX25 is very similar to the non-backhoe BX models, but not exactly the same. It has a swivel seat, hydraulic plumbing and reinforcements to the tractor chassis to accomodate a backhoe (beyond the backhoe subframe, by the way). Having owned a non-backhoe BX for about 10 years, I have looked at the little Woods backhoe for BX's more than once. Seems to be a decent alternative setup, but it's not the same as starting out with a BX25 to begin with. The lack of a backhoe is about the only regret I've had with my BX; it has been a reliable workhorse in every other respect.