They've certainly existed, but they've only recently become more of an issue in single-player games, because they're progressing beyond simple cosmetics into mechanics that fundamentally shape the design philosophy behind big titles. "Invading", in this case, more means that this more extreme form of MTs is making its way into single-player games.
The argument isn't that microtransactions haven't existed in them until now, but that they generally haven't been that big of a deal until recently when whole games have been designed around loot boxes and monetization schemes. Microtransactions are "making their way into single-player games" in the sense that *true* microtransactions are.