I...soon after the law was promulgated, “an estimated 200,000 Ruthenian faithful became Orthodox”...
That shows you how "Catholic" these people really were, that they would leave the Church over a disagreement about disciplinary matters.
There's long been a latent sense among some of the Eastern Rites that they feel closer to the Orthodox than to the Roman Rite. You'll often hear Eastern Rite priests speak glowingly of the Orthodox while disparaging Roman Rite Catholics. That has long been a problem.
Yes, there is a lot of rivalry and resentment coming from the Orthodox Church towards the "Uniates" (which is a derogative term to mean the "Orthodox who are in communion with Rome" or better yet, the Eastern Catholics). They take any opportunity of conflict with Rome to try to snatch the Uniates (Ukranian Greek Catholics, Ruthenian Byzantine, Russian Catholics, etc) back to Orthodoxy. Of course, there are many souls, who ignorant of the necessity of submission to the Pope for salvation, are easily persuaded given that the liturgies are pretty much identical and of course, there is also a strong sense of nationalism that matters.
Everything boils down to the dogma of Extra Ecclesiam Nulla Salus though, and the necessity of the submission to the Bishop of Rome for salvation which the Uniates believed above everything (among the many other political and economic reasons, I imagine), that caused the re-union of these churches with Rome back in XVII century, even to the expense of the risk of "latinization" to their rites. Doctrinally they are bound to believe in the same dogmas than the Latins, (after all the Catholic Church is truly universal), but they can and do differ as far as disciplinary measures such as the married clergy.