I tried searching for a journey from Heathrow Terminal 5 Bus to Guildford after 19:00 on Monday, and both NR and SWR journey planners show one bus departure at 19:43 and a second one at 19:55, both heading to Woking from which change onto a train for Guildford is possible. Looking more closely on SWR's results, the 19:43 departure arrives Woking at 20:08 and "continues to Guildford" while the 19:55 departure arrives Woking at 20:25 and "terminates at Woking".
The
SWR "Airport Links" page shows that the "Servie RA2" runs Heathrow to Guildford, and links to
an out-of-date timetable (dated July 2020) showing a departure at 19:37, but the
Firstbus Railair timetable page links to a
timetable dated January 2021 showing the 19:43 departure.
The
SWR "Airport Links" page also shows a "National Express Rail-Air Coach Link" that runs Heathrow to Woking, but claims that "This service is currently suspended due to Covid-19". It links to the
National Express Woking to Heathrow webpage which does not mention this specific service being suspended, but does have a general disclaimer that "We are currently running a reduced network" and "the information on this page may differ depending on your travel dates and any regional restrictions". Going further and using their journey planner to search for Heathrow to Woking on Monday returns no results, and Woking is not even shown as a destination
on their route map or in their
timetable search. A google search reveals an
undated timetable for route 701 showing the 19:55 departure from Heathrow Terminal 5 to Woking, but the 701 is not shown in the
route list.
So the most likely conclusion is that you were scheduled on a National Express Rail-Air Coach Link (route 701) service which is currently suspended due to Covid but is incorrectly shown in rail journey planners, and you were lucky to catch a Firstbus RailAir 2 departure around the same time.
Edit: Thanks to
@Deerfold below for the correction on the route number.
