It's not. They're free to choose who they trade with, using who that country trades with as a decider violates their autonomy.

You're just framing the violation as a choice and saying it is their right to make that choice. Sure, they also have the right to make the choice to invade Canada, but actually invading is obviously violating their autonomy.

Okay let's use a specific example:

I decide that I don't want to trade with Country A because they are producing weapons that they plan on using to attack me with. Now let's say Country B is trading with Country A, and I think they are trading components being used to produce the weapons in Country A.

Is it a violation of Country B's autonomy for me to decide to not to trade with them because they are trading with country A?

If I believe that anyone who trades with Country A is supplying them resources to build weapons to attack me, and I decide to not trade with any Country who is trading with Country A, am I violating the autonomy for all of those countries?

>Is it a violation of Country B's autonomy for me to decide to not to trade with them because they are trading with country A?

Yes.

>am I violating the autonomy for all of those countries?

Yes.

The clearer example would be imagine country A attacked me, would refusing to trade with country B violate their autonomy? And the answer is still yes, but both country A and B are already violating your autonomy so it's a justifiable violation.

With your example, I wouldn't say producing weapons clearly violated your autonomy, but that's a completely different argument.