Not all anticompetitive practices are illegal, but this isn’t even anticompetitive.

Anticompetitive practices are actions that reduce competitiveness in a market by entrenching your dominance over the (usually smaller) competition.

Not allowing your competitor to buy your product arguably increases competition? It pushes them to improve their own product to be as good as yours.

No it's not. It's only illegal if you are found guilty of it.

And for that to happen you need to be (a) an effective monopoly, (b) have a negative direct or indirect impact on consumers, (c) large enough for regulators to care about and (d) be in a regulatory environment that priorities this enforcement.