I am confident the endorsement of management was required for BHP to make the offer.  It assures them (BHP) that the board won't use the shareholder rights plan (poison pill) to try and stop or increase the offer price.  

Given the timing, and the fact that the price is just higher than the Wyloo one (should Noront have used the poison pill, I believe all shareholders would get 1.5 the value or .315*1.5 = ~.47 per share) - it was either endorse BHP at the higher price, or nothing, which guarantees a lower price even after enacting rights plan, from Wyloo. 

All speculation I'm far from an expert on these things, but if the shoe fits...