A point of clarification: Trump's public actions and proclamations are not an example of insider trading, which is done behind the scenes to benefit a specific few.
His actions are better categorized as market manipulation, including both his on/off threats about tariffs and his tweet about how it was a great time to buy. This isn't good. This is a multi-trillion-dollar behemoth and he is the leader of the world's wealthiest nation. If he were on the outside looking in, he would likely be outraged by it.
That said, I agree that it's a good time to buy during any downturns, as the market is being volatile but should ultimately continue its long-term upward trajectory... eventually.