Honestly I think this is something us humans are really bad at (well we're known to be bad at this). Understanding causal effects through time. Not only are we bad at understanding causal facts but we're really bad at being forward thinking and understanding temporal phenomena.
But I think the main point that the gp is making is that most of the things that happen under a president aren't really felt till well after they have left office. Many wins that presidents have are due to effects from former presidents. Like both Obama and Trump got praised for high S&P 500 values at the beginning of their presidencies. Obama came in after a big crash. There's also no way Trump could be a causal factor for growth in the first 3 months of his presidency. You can't really have that kind of effect that early on. Yes these presidents contributed to these markets, but it would be naive to say that the growth was because of them. Or we can look at the long lasting effects of Regan. Or even Civil Rights and Johnson. While there was more immediate consequences under both these presidents there were also very long term effects that we still see today.