I've read that too, but I think wether it's true or not is irrelevent. Everest says it is, and I think Jeff says it isn't, e.g. point 5 here
The purpose of the tool/page is two-fold:
a) before the room is built to allow you to design a room that is less likely to have overlapping modes. It doesn't matter if one mode is twice as strong as the other for one reason or another, if they're they same frequency they could add up (superposition) into something serious
b) after the room is built and measured, if a troublesome frequency is found, it gives a first hint as to where to place absorbtion to treat it. Sort of a time saver. It may be right or wrong (acoustical room dimensions vs phsycial room dimensions) but it's certainly a place to start, considering how vastly little thought and time it takes to find out which axial or tangental it might be, if any.
Would it make sense to make the lines shorter for non-axial modes?
So I don't think so. A couple of variations on line size/position I was considering were:
a) splitting the lines into three with a column for axial, a column for tangental, a column for oblique -- but I decided it's redundant information on the piano graph and that the list of frequencies on the left does a much better job
b) having thicker or longer lines when two frequencies overlap -- this is a lot of programming and again is redundant because the left list of frequencies has that colour coded information.