I was doing some back-of-the-envelope calculations just now to show how Io experiences much more tidal stress than Mercury due to a stronger change in gradient, when I discovered my assumptions were wrong, but I'm posting the results anyway:
Mercury's orbit has a huge eccentricity, and its distance to the sun varies between 0.47 AU at Aphelion and 0.307 AU at Perihelion. Taking Mercury's size into account, that should work out to a difference in solar gravity between its near and far side of about 0.000004 m/s^2 at Aphelion and 0.000013 m/s^2 at Perihelion! That's actually quite the squeeze!
Now comparing that to Io, which has often been reported as having this wild orbit, upon looking up the actual numbers I saw that Io's orbit around Jupiter is actually nearly circular: 420000km at Periapsis and 423400km at Apoapsis. The moon itself has a radius of 1800km, that means its orbit is so circular its variation falls within the size of the actual moon. Furthermore, plugging in the numbers, Io's gravity gradient between its near and far side remains pretty much stable at 0.000014 m/s^2 no matter where it is, varying only by a factor of about -1.37x10^-08.
So the gradient change itself is not dramatic in Io, but reasonably pronounced in Mercury - didn't expect that. Taking a look at the orbital period of both objects, Io completes about 50 orbits in the same time it takes Mercury to orbit once, but even a factor of 50 isn't enough to blow up Io's tidal action to make up for the observed difference in tidal heating.
My only conclusion at this point is that I either made a careless mistake somewhere (which is likely) or that Io's squeezing action must come almost entirely from its interaction with the other Jupiter moons.
In the end, a heat source is not required for Mercury to experience tectonic activity though. As described in the article, the cooling and shrinking of the planet itself are sufficient to cause disturbances on the surface.