Correct, they can't. But not for the reason you give. The EU is actually quite strong at standing up to the US. See just the recent retaliatory tariffs they imposed on the US in response to Trump's tariffs on steel and aluminium.

They are also strong against individual US companies like Google, Microsoft or Facebook. And I am sure you've heard about GDPR.

You may notice that in the Trump's trade war case the EU acted in a reactive way. Something happened and they responded in kind. This tactic is called tit for tat and, in game theory, is classified as one of the "nice strategies": never be the first one to attack. It may sound weak but numerous studies have been done to pitch "nice" and "nasty" strategies against each other and, although trigger-happy, nasty strategies usually tend to get a quick advantage at first, nice strategies always win in the long run.

The reason the EU cannot do much about the US-based OS hegemony is because they cannot go to Company X and tell them, "thou shalt produce Product Y". That would be dictatorship.

Nor can they go to Company X and tell them, "though shalt not sell to Company Y". That would be acceptable under anti-trust and anti-monopoly laws but such things can be and are done at national government level.

There are only two ways the EU could make a non-US mobile OS happen:

1. Set up a company to do that. Which, as kinggo correctly pointed out, would be hugely inefficient and most likely doomed to failure. Partly because of the incompetence kinggo has mentioned and partly because of the (justified, IMHO) mistrust of the general public to put their devices in the hands of a government.
2. Pick a company that is developing an independent OS and support them. To be fair, I do not know why they have not done that with Jolla, I assume Jolla never asked. But even that would not guarantee a success. Competitors might (rightfully) see it as an unfair advantage, which would break the "never strike first" rule.