BU will let the user select a given Core or XT BIP (this is still be worked on (BUIP002, probably not supposed to link it here)), so for example if they turned on the BIP101 option, their node would mimic an XT node as far as following BIP101, including the 75% threshold and specific starting block.
Really? How? So far what I have seen is that a new block size limit in BU takes effect immediately. There is no mechanism that does the supermajority fork process.
I think you overlooked Zangelbert's mention that the BIPs are work in progress through BUIP002 - a BU Improvement Proposal.
You are correct that there is no full emulation of the BIP101 threshold etc. for now, and I believe the matter of to which degree BIPs need to be emulated faithfully is still being discussed.