Is plate tectonics a quantitative theory?
That it is still a qualitative theory and not quantitative as yet.
We have a very good understanding of how plate tectonics works in a general, qualitative sense.
What is missing however, is a quantitative model that would allow accurate predictions of things like: plate motion or when earthquakes and volcanic eruptions would occur.
For example, in astronomy, we have both a very good qualitative and quantitive model (theory) of the motion of the planets around the sun. This model is so good that it is used to chart the course of spacecraft to the moon, Mars, and to the edge of the solar system. We are not quite there yet with plate tectonic theory.