the onboard battery controller is responsible for determining an agreed upon charging rate. The thread you linked had an adapter that could output 9 and 5 volts as well.
I believe a 60 watt charger is the minimum wattage to charge while using the device. Anything below that will help to offset the battery discharge rate, IF the battery controller and your power source agree on a voltage and amperage.
But I’m not entirely sure of that, because I have a battery pack that will charge at 12V @ 1.6 Amps which is just under 20 watts. This charges the laptop at a rate of about 1% every 10 minutes, but this really just depends on load.
That said the world of PD USB C is wonky. It could be a lot of things.