Okay, I was assuming that they are actually connected, since “interstellar highway” is the only option available. I guess that’s a bug, then.
Note: I just sold my off-road licence and I can confirm that it is possible to use the “interstellar highway” option to travel between these two star systems.
Also, there are more connections that just this one.
Also, when you attempt to use a highway to travel between constellations when the highway option is greyed out, you get the same message as with star systems in the same constellation (which tells you that “you must first explore all star systems along the interstellar highway”). So you should probably change that to avoid confusion.