You can see that it doesn't like the use of \DeclareMathOperator*{\argmin}{arg\,min} there in katex.

i.e. it is

$$
\DeclareMathOperator*{\argmin}{arg\,min}
\beta_0, \beta_1 = \argmin_{b_0,b_1} E[(s_0(x) -
b_0 - b_1 (S(x)-E[S(x)]))^2]
$$

But I beleive that DeclareMathOperator might only be valid in the preamble in latex and this shouldn't actually work.

Regardless, I think that either the DeclareMathOperator needs to be centralized in one way or another or (preferably) we just skip out those sorts of preamble things and have it raw. It doesn't appear that DeclareMathOperator is used anywhere else, so this might be the only location.