I considered that interpretation before writing and I found it impossible to read that way. For a start, "imitations" is plural, so no, it doesn't just refer to

I considered that interpretation before writing and I found it impossible to read that way. For a start, "imitations" is plural, so no, it doesn't just refer to Jupyter. And "started" certainly implies that other imitations hadn't appeared before that. While I'm at it: he deserves an uncharitable reading, because he's having a dig at Jupyter (which may be an imitation of Mathematica notebooks) and neglecting to mention any influence of previous work on Mathematica notebooks, such as Knuth's literate programming.