“Vulnerable to the US” can mean so many things that it practically loses all meaning. Even with a launchpad in continental Europe, they have many ways to interfere with this project, from disrupting the supply chain to carpet bombing the launch site.
replyA ship crossing the ocean is much more vulnerable than a launch site with sophisticated air defences.
replyThe US can also decide to “free” and “bring democracy” to the French Guiana because the rich oil deposits of its continental shelf won’t free themselves.
The point is that a slow ship crossing the ocean with hard to replace is very vulnerable in transit.