Zygarde is a particularly interesting Legendary, as it developed considerably between its first appearance in Gen 6 and its return in Gen 7
As such, the format uses the First Edition rule book (which is not nearly as daunting or complicated as you might imagine), and has no floor rules outside of that slim rule book
Counter the bombs, apologizing for it but explaining that if you didn't stop it then the game would've been over
Also, the fact that you have to bounce 2 islands to pay the alternative cost, or else pay the full 5 mana screams "late-game play" to me
this is true of the Mystical Archive cards and the alt art planeswalkers from War of the Spark
While it seems uncomfortable in decks with too many colors, tapping for colorless and filtering mana to any color should make it an easy inclusion in decks with one or two colors that want this effect on a land