I don't believe anyone is saying there isn't a difference.
I believe most are saying... in the scope of how music is used for DJing purposes, people simply cannot tell the difference between the vast majority of mp3's as to what bit rate it's ripped at..or what type of file was used.
Yes a song can be ripped poorly...just as it can be recorded on a CDR poorly....or even recorded poorly on a master.
Equipment can be poor. EQ can be poor. Volume can be inappropriate. So many things in the sound chain. Extremely difficult to put blame on bit rate as a guest if they notice anything at all.