Ken, you got it correct. Salts in the sparging water do not cause that water's pH to drop. pH drop is only a product of the phytin reaction in the wort.
While that sparging water will end up in the mash, the resulting wort dilution and replacement with sparging water means that there ends up being very little phytin in the mash to react with. So we can't even rely on those salts to keep the pH low during the sparging step.
Regarding the need to acidify RO water for sparging use: No, you don't need to acidify. The primary thing of concern with sparging water is that it have low alkalinity. Good RO water already has very low alkalinity. So the need to acidify is gone. Another consideration is that with the very low alkalinity of RO water, it would only take a drop or two of most acids to cause the pH to plummet.