Their point is that the closing a road once doesn't just make all the pollution not happen, it time-shifts some of it to other days when people are driving again.

I expect there are some people who do make other arrangements and take a bus, so it's a net gain, but there are also those who take their car on Monday since they couldn't on Sunday.

EDIT: I looked up how RideLondon works, map at the end of this PDF shows road closures https://d1ffaecguugkl4.cloudfront.net/ridelondon/live/upload...

So it's not even time-shifting necessarily. People can drive around that and just move the pollution from one street to the next. But yes, the pollution measurements on one of the closed roads would look better that day.