It took roughly 20 years for the EU to deploy 30 satellites.

How long do you think it's going to take to deploy 300 or so of them when the Ariane 6 had only one launch (with a partial failure) in the last 14 years?

If you want to build a constellation, you need the means to send payloads in space at a relatively low cost. The EU can't do that so it will be an expensive and slow endeavor and by the time those 300 satellites are up there, Space X will have deployed 10s of thousands of them.

You can say, that the EU does not want to compete with Space X or that their goals are not the same but either way, it's just too little too late IMHO.

I agree with your overall point, but that's a super deceptive metric to use. First Ariane 6 was scheduled for 2020 and only the first one ever launched.