That's just the point. The magic connects one portal to the other. You don't go through anything but the two portals. There is no Regio or other realm involved. Just a funky alteration of two locations so they connect to each other in a way they normally wouldn't.
Look for me I guess it comes down to how I want to describe a Hermes portal in operation. Lets take the example of a Hermes portal connecting the Levant and Stonehenge tribunals.
If someone comes and activates the Levant portal and through the opening they see the green fields and blue skies of England, and when they step through it's just like stepping through any other doorway, then I'd call it space magic and the person traveling through doesn't have to worry about warping or penetration.
However, if when a traveler activates the portal they see a shimmering blue energy field (for instance) and the trip is described as some sort of psychedelic magical water slide ala Stargate, then I'd start to worry about teleportation, regios and realms. (And the attendant warping and penetration issues)
Obviously I prefer the "space magic" explanation. It really seems to be a better fit for my understanding of Mythic Europe.
The Target (as in R/D/T) of the spell is the thing created/affected. A fire spell with T: Group creates a group of fires. The fact that these coincide with a group of knights location (in which case their potential MR would protect them) is incidental.
More exactly: He would not resist the secondary, mundane fires that could have been created by the first, magical one.
But yes, he'd resist a magical, continuous, fountain of fire if he threw himself in it, even if it was created 3 days ago.