I was told the Latin phrase is actually "for the many", and that "the many" means "all" in Latin. Am I mistaken here or what's the story?
He's referring to the Greek where many with the article, so "THE many", means "the multitudes", which could be rendered as "all". Perhaps you've heard the expression "hoi polloi". Only problem for this theory? The Greek version given in the Gospel of St. Mark LACKS the article "the" and simply says "many". Many in Greek without the article can never mean anything other than many. Lots of these glib little answers from Novus Ordo-ites who haven't actually done the research.
Another answer was the claim that semitic languages lacked a separate words for "all" and "many" ... which has been long debunked as a lie. Not to mention, you're claiming that the Holy Spirit in rendering the Greek (it is the Greek text that the Church holds to be inspired), happened to chose the wrong possible translation of the semitic, an error with Bugnini and company were able to correct the Holy Spirit about ... nearly 2000 years after Our Lord said the words.
So, since both of these reasons are demonstrably garbage, WHY did they mess with this? Was it just to make the Church seem more welcoming to all of humanity or was it to deliberately invalidate the Mass ... just to make sure?