A flight instructor does NOT need to act as PIC in order to flight instruct.

However, the acting PIC DOES need a medical in order to be PIC.

Thus, a CFI without a medical does NOT need a medical in order to instruct a pilot that is qualified and able to act as PIC.

The easiest way to explain it is a CFI without a medical can instruct IFR stuff in the clouds, but not with the client under the hood, and no student pilot instruction.

Next, a flight instructor is subject to limitation on their duty day, just like an airline pilot or a mechanic. It's no more than 8 hours combined in a 24 hour period of flight instruction time, part 121 or 135 flying time, and/or "work time" as a mechanic.