I believe he is speaking in general terms - christianity vs other religions, not specifically the Catholic Church. Full Unity , is the work of the Holy Spirit.
Christianity makes a series of claims about God and man: That Jesus of Nazareth was God Himself, and that he died and was resurrected — all so that we might be free from our sins. Every other religion in the world denies each of these points. So, if Christianity is correct, then it speaks a vital truth to the world — a truth that all other religions reject.
Respectfully, I’m not sure I’m properly understanding what you have said. But I think you are saying that some of the Protestant religions profess some things that are truthful. Perhaps they do. But they also profess many things that are absolutely false. Indeed, Martin Luther certainly professed those same truths that you’ve just articulated. He was a Christian until the day he died and the Church excommunicated him as a heretic. The Orthodox religions also profess those same truths as well, and the Catholic Church and those religions have been struggling with their differences for over 1000 years..
The issue as a Catholic, should be something other than trying to find how other, heretical religions are actually pretty good religions too. That’s modernism. The Church used to teach that there is only one true religion––the Roman Catholic religion. Certainly neither you nor the writer would deny that. Why should any Catholic be concerned about the pluses and minuses of “Christianity” vs other non-Christian religions? That may be an interesting academic discussion, but it is one that might better be had in a secular theology class. The purpose of his writing was to tell Catholics what they should know.