Insider trading doesn’t exclusively apply to employees of a corporation; if it did, execs would be giving tips about major decisions to their close friends.

The point of regulating this is that trading based on exclusive information that the rest of the market can’t possibly have is unfair and undermines trust in the market. Trump (allegedly) tipped off his allies about a future market-manipulating decision before it was announced to the rest of the market. They made trading decisions based on that exclusive information, and profited off of other traders’ lack of information.