That makes a lot of assumptions, especially given there was most likely Y2K problems that needed addressing and system upgrades taking place since the initial system rollout. This page[0] states that there was changes to the system in the 1960s but I can't find anything more recent so... maybe you're right. Who knows.
Y2K problems were related to attempting to represent years with 2 digit numbers. By no means would setting a date to 1875 fix a Y2K problem, it would just exacerbate it. Even if the problem originated from a Y2K "fix", it would still be a major bug, resulting in millions to billions in excessive payouts going to who knows where.
I think parent post is saying that when they fixed the Y2K problem, they likely also modernized the date representation to comport with international standards. It wouldn't really be any more difficult to do both of those at the same time than just fixing Y2K bugs.
[0]: https://www.ssa.gov/history/ibm.html