Chemistry - Acidity of hydrochloric acid in acetic acid

So basically I am asking, is this value totally wrong or is it actually quite reasonable?

It's quite reasonable. Compare to page 32 of this reference.

This was much higher than I expected. Also doesn't this imply that
Cl− is stronger as a base than HCl is as an acid in acetic acid, which doesn't sound right.

To answer whether Cl- is a stronger base than HCl is an acid, you need to know the self-dissociation constant of acetic acid, the equivalent of Kw for water. It is about $10^{-10}$. So whereas the middle of the pH scale is thought of as 7, the middle for acetic acid would be 5. So, yes, Cl- is a stronger base than HCl is an acid in acetic acid.