At least my Sherpa 100AC (with up to 3A output) is able to power my PPM and charge it very slowly even in presentation mode. The battery charging can charge at around 30W according to my measurements, so the total power consumption seems like it could go above the 60W (theoretically 70W, but the input current from the USB-C port is limited by the charger chip, reducing charge current to stay below the max). I haven’t figured out exactly which current it limits to (the TI datasheet only shows that it’s possible to limit the total amount of current). I think it’s limiting to either 3A or 4A, likely depending on the negotiated PD contract. I haven’t scoped those signals out yet.
You’re better off if the “15V” output is slightly higher than 15V from your power bank, like the PPM adapter that outputs 15.5V. This is because the input limiter is purely limiting current, not power. The TI BQ chip used as the charger does have a maximum input voltage of 17V (@Philips_Support_P mentioned a while back that the PPM would refuse to negotiate a 20V contract because of some of the chips not handle it, and indeed the limit is 17V).
Edit: Not all powerbanks/adapters supporting 15V/3A manages to negotiate a contract with the PPM as have been mentioned several times in this forum. Most of my PD adapters handles it but my favorite car PD charger (the Mikegyver https://mikegyver.com/product/mct60w-mikegyver-macbook-pro-touchbar-usb-c-car-adapter-charger-external-batteries-5/ ) unfortunately does not complete the negotiation. I’ll see if I can figure out why at some point.