That's correct. It is a line isolator. Your antenna will need a choke or line isolator. For your antenna, just get some 61 ferrite material and wrap a few turns around the core and get some slip on beads.
The most power I run on any of my loops is 20 watts. I had a bigger tuning cap and I could run up to 100 watts, but I could not get a low enough value to go above 20 meters. I did not want to use a vac vari so I used a small dual section air cap, wired it in series to increase the voltage breakdown value and get a lower capacitance so I could get on 12 meters.
Remember, 20 watts is only a tad over 1 S unit down from 100 watts. You save on battery current drain, you are exposed to less of an RF field and you can use a smaller cap, meaning you don't need a vac vari.