[Park Seward]

You are missing the point. If you have two discrete inputs, where the line input joins the channel strip after the preamp, then yes, you have a true line input.

I don't believe I am missing anything. The combo Neutrik jack has different connections (two discrete inputs) for the XLR and the 1/4 part of the jack, just like two different jacks would. One goes to the preamp, the other after the preamp. You are making different connections to different circuits. The difference in input impedance shows that.

However, these days, and particularily in the low end of audio, the line input is simply wired behind the microphone input jack, with a pad to allow a line level in. It's a cost issue.

In this case, you are still going through the preamp. And since the trim pot is part of the preamp, the "line" signal is also attenuated using this control. Which will reveal to you whether or not you have a true line input.

True for some implementations. However, there are two jacks in one connector. Check out the Neutrik wiring diagram.