I think the opposite because in this specific case we have the US which has the world's strongest navy, and the strongest navy by far in the Atlantic.

My opinion is that there is an implied reliance on the US "allowing" Europe to ship the rockets to Kourou and launch them there.

If our relations ever completely sour, the US might, for example, take over the shipping vessels going to Kourou, kinda like Europe takes over Russian shadow fleet vessels.

Now all of a sudden Europe has no means of launching rockets, except now it can launch to polar orbits. It might seem a stretch, but there is a non-zero chance that could happen, therefore it is better to have an alternative.