Title text:
The inflection point was probably in late 1966 or 1967, so when Neil Armstrong flew to space on Gemini 8, plate tectonics was not widely accepted, but when he landed on the Moon three years later it was the mainstream consensus.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3159/
Of course people hypothesized continents have drifted, but that wasn't widely accepted until a mechanism of how that could have happened was proposed. I don't think xkcd was suggesting that nobody had the idea of continental drift before 1967, just that it wasn't the consensus.