That's not true at all, unless they mean the crossover wiring between the front door speakers and the dash tweeters.
The reality is you can replace any speaker and use the stock amp just get speakers that have high sensitivity (don't require a lot of power) because the Infinity amp is not exactly a boomer!
'95 ZJ Ltd. White 4.0L
TW DC DS; ARB Bull Bar, F&R Air Lockers; Superior Axles; Warn Winch; 4.56 gears; ACOS, Addco F&R Sw Bar, JKS Discos, Rr Track Bar & Nerfs, 249>242 Swap, Stillen rotors+Kolak pads, 3 pc. Armor, RE 4.5 Kit + 2.5"BB, Adj. UCAs&LCAs, OME SS, Bilstein 5150s, 15x8 AR Diamond Backs, 33 Truxus MTs, K&N FIPK, RE 62mm TB, Perf. Distr. 4.0L Kit, Exhaust: 2.5" Kolak exhaust FM 70: Optima Yellow, Mean Grn Alt., PIAA 520s, 540s, Kevins Rack, & TB Conversion