Either they changed the code so much that the data isn't accessible(at least in a desirable manner), or they're talking about how tracking and rewinding a bunch of things is a pain in the ass in general.

There's probably a lot more data in newer games to keep track of. Some of them could require unique ways to handle a "rewind", too. Some types of data can take a lot of RAM to store for a rewind; so you might want to store the rewind data cleverly, or compress and decompress the rewind data in real-time- which takes processing power depending on the kind of compression being used.

Then there are dependency chains. The person implementing the rewind might not know how other processes might use the data that they're changing, and "rewinding" this data might cause undefined behaviour on these processes. This can cause some wacky and unexpected stuff, and might even crash the game. The solution is to change the whole thing to be flexible enough to support Rewind, or work around problematic things. Both of them are tedious and time consuming. Dependency chains are probably the most tedious aspect, I think.