pluto has a really weird orbit that brings it in between uranus and neptune, and actually takes it closer to uranus from time to time. there's a large, measurable effect of neptune on pluto's orbit that is well understood. accepting that the geological activity may be due to gravitational effects that planetary mechanics refers to as "oscillations" is still a little outside the realm of accepted science, but it's also a research question that is building greater support. it seems as though planets like jupiter and mars may have some effect on the geological activity that occurs on earth. likewise, the answer to this apparent mystery may very well lie in this developing understanding; it may be gravitational interaction with the other planets, primarily neptune.

i see they're ruling out tidal drag because of the perception that charon and pluto are in equilibrium, but this is a goofy assumption for three reasons:

1) those calculations are fairly crude. i wouldn't be going around using them to argue against anything.
2) "equilibrium", in context, is never going to be "perfect equilibrium". there's outside forces. it's not a closed system.
3) the larger pluto-neptune-uranus system orbits in cycles that are measured on the scale of centuries. we don't have the kind of data required to draw conclusions about the stability of the system, and won't until we can at least observe an entire cycle with what we today call modern equipment.

it's consequently more accurate to say something like "plato and charon seem to be in approximate equilibrium at this phase of the uranus-neptune-pluto system". and, that statement doesn't say anything about the possibility of tidal drag through other phases of it.

the way i'd imagine it is something along the lines of that pluto is moving through these gravitational fields in a relatively erratic fashion and that the modulations that occur as the result of that movement are exerting pressure in different directions. for a large part of the orbit, pluto would be held in place by a four-body system dominated by itself, the sun, neptune and uranus. it would often be pulled in opposite directions by uranus and neptune. oscillations in that system could produce the forces required to set off the plate mechanics.

but, i don't really have the interest to work this out in detail. at least not right now, anyways.