It does acknowledge, though, that this could only be achieved for narrow bodies, being currently too much of a safety risk for wide bodies
It also points out some other inconvenient facts:
a) No aircraft larger than the A318 has been certificated for a 5.5 degree approach
b) The A318 itself could only be certificated following mods to the flight control system and spoiler deployment, and specific training for crews in using the technique
c) ICAO permits steeper descents only for obstacle clearance reasons, not for noise abatement.
Steep approaches at Heathrow are a non-starter.