This proverb is noted by two of the late paroemiographers: Macarius Chrysocephalus 5.85 and Apostolius 11.30, but each gives it slightly differently from each other and from the Suda's version.
What the Suda's gloss (Apostolius has a different one) presumably means to say is that only after the Persian incursion at Marathon (490 BCE) and again, in "the war" (the second, larger Persian invasion of 481-479) do the Greeks make a successful anti-Persian alliance. But this reflects a shaky understanding of the period in question, since the alliance that faced the second Persian invasion included a wider array of Greek cities than the alliance of the "Delian League" that persisted afterwards.