[kathe-rine]

The RER brings riders from the suburbs into the city, but doesn't add to the infrastructure for transporting them once they're in Paris.

[dorfsmay]

There are RER stations within Paris: everything within zone 1 is in the city of Paris:

https://parismap360.com/carte/image/en/paris-rer-map.jpg

But even if cutting prices is within Paris only it will probably affect suburb-Paris traffic, because it changes the cost of a trip to Paris, possibly now cheaper than driving. The article is light on details, but it looks like these measures are taken by the city of Paris with the aim to reduce the traffic within the city.