I see what you're saying, but I think it's a misunderstanding. 1D here only means that there's some state where the active cells are confined to one row — but one row within the ordinary 2D GoL plane. I'm sure the next iteration leaps off the line immediately. Search for "Blinker" here to imagine how it could start spreading off the line.

https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life