The décret establishing the list of social networks forbidden to the <15 yo will be appealed before the Conseil d'État, which will most likely send a question to the CJEU and have the ability to cancel the décret, so I would argue that their opinion is extremely relevant. Same thing for the eventual sanctions taken by the Arcom.

European law takes precedence over national law, so the effectiveness of the bill appears to be limited. The same thing happened with the French age verification bill for adult websites.

I don't know how it works in France, but in common law systems opinions of other courts are always at least capable of being persuasive (not binding) precedent, so they are not irrelevant. Other courts can be, and often are, influenced by persuasive precedent when appropriate.