Maybe its a typo, but from the measurement of the individual branches I would rather suggest that the lower bass is cut off at 900Hz rather than 90Hz as was stated in the description.I would argue that the BBC dip originates in the back then new 2-way paradigm with larger plastic woofers (8") and tiny tweeters (3/4") with, by todays standards, very high x/o frequencies (3,5k). The on-axis equalization might have compensated the directivity error.
The BBC dip is overdone here. Again, it may be some 'voicing' as to discriminate the product against others. For many such minor differences make or break a deal. If all were the same (perfect) what to decide for, having only the naked ears to evaluate performance?