Can someone explain why this pJFET claims Rds(ON) of 270ohms to be "low"?
When the JFET is on, shouldn't we expect something like 0.003 ohms to allow maximum current through the open drain/source channel?

As I understand it from the tutorial, when the JFET is off (saturated gate/source junction), then the resistance should be high with a closed drain/source channel.

 

There are some JFETs with much lower Rds(on), but I don't think you will find any that come close to the big MOSFETs that sport on resistance measured in milliohms.
The JFET I linked to is spec'ed at 5 ohms. There may be rower resistance devices available. I found this one after a quick search.