is it just more selective filters in the receiver?
Mainly narrower filters in the transmitter section. As I stated before: if one transmits on, say, 119,000 Mhz with a 25 KHz filter than one will also be received/heard on 119,0 - 8,33 and 119,0 + 8,33 - which is not what is intended.
But also narrower band filters on the receiving side, for the exact same reason, mutadis mutandis.