Prove it with sources.
There's no DMG guidance on how Polymorph affects offensive and defensive CR, or how Hypnotic Pattern or Tasha's Laughter affects offensive CR. It's just up to the DM to guesstimate something.
Aside: "They" has a 700 year-old history in English as the gender-neutral pronoun. It is perfectly correct to refer to anyone as "they" if you aren't sure. Obviously once you know better you can be more specific, but there's nothing wrong with "they" as a wildcard.
"Eche of theym sholde ... make theymselfe redy." — Caxton, Sonnes of Aymon (c. 1489)