Hmm
Ironically, quite a large part of the route looks like it will disappear if the proposed Heathrow extension is implemented!
I think they might have at least a few months of operation before that's an issue.
And I don't quite follow how a Euro 6 compliant bus emits less than a much smaller Euro 6 compliant light passenger car either.
The mention of a "single occupancy" car suggests they were thinking about emissions per passenger (whatever they did or didn't put into the words). In g per vehicle-km the exhaust emission numbers look like:
Car: (NOx) 0.06 (PM2.5) 0.0015
Bus: (NOx) 0.6 (PM2.5) 0.0023
(Other emission factors are available. Come to that, emission factors in g/kg fuel are available too.)