We may think of an inverter as a functional logic block, but it is after all nothing more than something which puts an output on the end in response to an input on the other. It is essentially a little push pull device. Parallel a couple of them for current handling, and feed them audio, they will drive a load with it. We see that here and there. For that matter I have an old Laney something or other here that uses a 4069 CMOS IC as a preamp stage.
Factories modify things all the time. They discover that an amp has undesirable charachteristics under some circumstances that they never planned for, so they make a change. If they added series caps to those two resistors, I'd assume it was to block any DC oproblems that might occur between ICs.