It varies depending on which channels you’re looking at. The Twitch average deviation between viewers and users in chat is usually much closer to 20%, with some as high as 50% and rarely below 5% difference.
This difference also varies greatly depending on the type of stream happening.
Those figures are just a very rough overview based on doing ~6 years of Twitch analytics myself.
tl;dr don’t assume the chatters endpoint is anywhere close to actual viewers.