It is not that a signal transformer may not pass low frequencies...

The low frequencies (depending on frequency, on primary inductance and mic's output impedance) never arrive at the primary as they are dropped right at the mic's internal output impedance.


Of course an electromagnetic music wave travels from mic to transformer primary winding at the speed of light in whatever conductor transmision line. So the mic does not know yet the bass frequencies will be sent back by impedance mismatch reflection.


It is not a bad thing as it may aid towards flat frequency frequency response from singer's or producer's brain to listener's brain. In short bass or mid heaviness can be reduced.


The primary winding resistance plays the important role of adjusting the high pass (ie low cut) slope to a gentler one so that the sound is neither thin nor bass heavy but natural.