BU does not always follow the longest chain.  In general BU operates in the same way as in Core,  BU nodes follow the longest valid chain.  There is a slight difference with respect to the blocksize issue.  It is possible BU nodes will follow the longest chain, where the blocksizes are arbitrarily large, but only under certain limited unusual conditions.  There is an “N depth” idea in BU, where nodes switch from regarding one chain as valid to another chain, if the chain with larger blocks has a lead of N blocks.  The default value of N is 4 and N could be manually set to any number, including infinity, when BU nodes will never switch to the longest chain.  Only if N is manually changed to 1, does BU follow the longest chain with respect to any blocksize.  Therefore the claim that BU always follows the longest chain is false.