I just got a chance to read the cardinal’s book yesterday, so can comment somewhat more informedly upon the issue, at least from his perspective.
Cdl. Stickler does not claim those councils to have imposed continence upon every cleric of any rank whatsoever (thus filtering down to your exaggeration of altar boys), rather, just as you said, those early councils imposed continence upon all major orders, with the subdiaconate as of then still being inconsistently classified as major or minor. In imposing that continence (that is, in forbidding both the contracting and/or the use of marriage), the early councils of the fourth century believed themselves to be reinforcing the previous discipline of the Church - i.e. not creating a new regulation - and enforcing an Apostolic discipline. While Cdl. Stickler seems willing to place solid trust in the Apostolic origins based on this witness, I would more cautiously say that while North Africa in particular was an incredibly traditional area, we cannot absolutely prove from this assertion any more than that the churches knew clerical continence to be of immemorial custom (apostolic being what they tended to call practices they couldn’t remember doing any differently).
Cdl. Stickler’s argument is that it is outright foolish to conclude, based on what we know of historical legal practice, that laws only came into effect upon being codified in written form. He is absolutely correct that ancient (and even medieval) civilization was governed by oral laws and traditions that could go centuries before ever being written down. To shift the focus later, to where I know a bit more, the role of a medieval legislator like a Carolingian king was not to make new laws - that was an absurd idea in a traditional society - but to determine and regularize practice based on traditional, often oral, law and enforce that. To put the cardinal’s point in an analogous, though qualitatively different way, let’s look at some of the ecumenical councils held in the fourth century. Before 325 we have no ecumenical councils that say Jesus Christ is God. How many Catholics or Orthodox would argue that, since this is the first statement of worldwide Catholic belief on the matter, that Catholics were not bound to believe in the divinity of Christ until 325? Wouldn’t we rather say that the Church had always believed in Christ’s divinity and was only now forced to clarify its thoughts based on novel, popular, and persistent denials of that belief? Now, Cdl. Stickler is not trying to claim that clerical continence is like fiath in Christ’s divinity, but he is saying that what we see in the early local council’s legislation on celibacy is the reassertion of a longstanding position (in this case a discipline) in the face of a challenge perhaps not completely novel and yet now popular and persistent. The bishops of those councils affirm their desire to reinforce (or re-enforce) a discipline that as far as they know had always been “on the books,” if we take those books to be oral law.
You are right, however, to say that if the rule were being overwhelmingly obeyed, there would have been no need for its reassertion, but if we are going to disqualify any ecclesiastical discipline based on widespread disobedience, well, we might as well say the Church has never had any disciplines (she certainly wouldn’t have a 1983 Code of Canon Law)! The Church has been plagued by sexually active priests even since the incontrovertible establishment of clerical celibacy, and even today in places such as Latin America has to deal with a large amount of concubinage, and this after at least a millenium of discipline against that very thing. That doesn’t mean the Church has no requirement of clerical celibacy/continence, it simply means we have further proof that human beings sin.