Make the claim, but don't spend the money before you get it. A subsequent delay caused by an ATC delay to a previous flight (as the direct causal link) would be considered exceptional, and not eligible for compensation. See here (ECJ case) and here (other ECJ case).

I don't know if the cause was ATC delays, but Italian ATC was on strike on June 4th and looking at flight paths between ATH and CDG I can see that could have an impact: https://www.flightradar24.com/data/a...y-ruu#30932627