Subject: Re: Voyager(s)
Yes, it is somewhat deceptive. People think of a really really hot furnace (for example), in which the probes would sublimate in an instant at 50K kelvins.

Basically, it refers to a plasma temperature at the boundary. However, the plasma is so thin (as you said with your mean-free-path), that it isn't really a barrier, or even damaging. It's -in essence- the energy of the plasma constituents, but they are very spread out.