Opponent uses Fusion Substitude, which reads "Fusion Summon 1 Fusion Monster from your Extra Deck, using monsters you control as Fusion Material," to fusion summon using monsters FROM HAND
https://duelingnexus.com/replay/ddb23502-109e-4d8a-a9dc-92aecade9237
#Fusion Substitute
merry hemlock
zenith falcon
Putting rarities on cards confuses the game and it probably thought this was the OG polymerization