In the PL I think they stated that any player that has played in the last year for the senior side counts towards the squad that can be available. I'm guessing it is the same for EFL Cup?
Any player meaning senior, reserve, youth team etc. If they've played even 5 minutes they count towards the eligibility. I think @RondónEFC mentioned it when Everton struggled last month because they'd used very inexperienced pros in the year and now they counted towards the squad allowance per se, which means it was harder to call the game off.
I don't agree with that. Like @Storts just said, so many other clubs have been able to get games called off and I don't see why Liverpool should be any different if the circumstances are the same as what other clubs have suffered. The timing is probably just coincidence of AFCON players going. There's a precedent that has been set by PL/EFL/FA etc including injuries in the availability quotas. Wasn't it Wolves recently that had 1 Covid-19 case and plenty of injuries and still asked for postponement?