Of course they knew how to use it. But I don't think they wrote the first lines of ed in ed-on-the-live-system. It is much much more likely that they wrote them offline and imported the code from punched cards or via tape or whatever from another system.

For whatever it's worth, Wikipedia (https://en.wikipedia.org/wiki/History_of_Unix) says: "In about a month's time, in August 1969, Thompson had implemented a self-hosting operating system with an assembler, editor and shell, using a GECOS machine for bootstrapping." I'm sure there are more details to be had somewhere.