Interesting. According to the RVTowingTips article, both of these guidelines (slab-side trailer and boat-shaped trailer) are aimed at reducing wind-induced sway. As noted, the TM is more boat-like, so you can make the case that the second guideline is a good one for TMs. TMs do have a longer coupler-to-axle dimension than other similar-sized trailers, and it does indeed seem to control wind-induced sway very well.
However, Leon mentioned another situation where wheelbase matters, and it has nothing to do with wind or sway. In this case, the situation is making a tight turn on a steep downhill grade. The trailer will try to push the rear of the tow vehicle sideways, instead of having it follow the front of the tow vehicle. This puts the tow vehicle in an over-steer situation. In a panic stop (or if you lose your trailer brakes), this could jacknife the rig. Ouch!
In his early days on this board, Rocky Mtn Ray, who is our towing expert, related his experiences with this situation. At the time, he was pulling a 2720SL with a short wheelbase Jeep of some kind. And it scared him. He mentioned this several times over the years - one of them is down near the bottom of the second page of this very long thread (post #55 in the thread).
http://www.trailmanorowners.com/foru...ead.php?t=2287
If you tow a trailer that's 6 feet longer (and 1000 pounds heavier) than Ray's, behind a tow vehicle that is only 6 inches longer, you might expect to experience some of these same problems. Part of it depends, of course, on where you plan to tow. Western Colorado is quite different from Indiana, say.
I have no idea what the "guideline" should be for this situation, but it does suggest that you want to be cautious about putting a long heavy trailer behind a short wheelbase vehicle.
Bill