Humans walk at roughly 2.1-3.0mph. "European cities" are listed as having bus stops 984-1476 ft apart, which would imply you'd typically walk half that to reach the nearest one (492-738 ft), which for a fit 3.0mph person is 2-3 minutes, and for a frail old 2.1mph person is 3-4 minutes.
Of course, people can be further away than that (they live orthagonally to the bus route), but you get the point. If you doubled bus stop distances to 1476ft apart, it would not add many walking minutes for the users.
Bus users can compensate for extra walking time by leaving earlier, provided the bus is on time. Good bus services can estimate arrivals in realtime, and show it to users on websites, apps, etc. as well as at the bus stop.
Bus punctuality is affected by a number of factors (e.g. traffic congestion, temporary and dedicated bus lanes), including number of stops.
The faster a bus can complete its route, the higher the route frequency can be with the same number of buses+drivers, which means buses pick up passengers more often, which means fewer passengers per stop (because less time between pickups), which means faster boarding, which in turn allows for a higher reliable route frequency. Having payment schemes like tap on/tap off, and having multiple entry doors also improves boarding times.
And that doesn't even consider that a faster bus route means you need fewer buses to run the same number of trips, so you can either run more trips (and save even more time for riders waiting for their bus) or cut down costs for the transit operator.
One can calculate how much area and thus passengers the stop covers and calculate walking times.
It's not completely trivial (with longer distance people chose alternatives), but can be done similar to the way the whole study was done with similar accuracy.