Earlier quoted context omitted.
Blog post author here. The kerning is probably a problem with their font config that can hopefully be fixed. I don't think -- and that's me personal estimation -- that people will write tons of `a\,=\,b` to work around the bad kerning, so that's a change I would still recommend making.
I thought kerning specifically referred to small adjustments in spacing between pairs of letters. This just seems to be a failure in honoring the spacing for the different TeX math mode symbol classes. The rel and op classes each have a certain amount of spacing they are supposed to have on either side (unless they aren't used as binary operators), and somehow this is broken in their implementation -- the spacing doe…
It was actually a config issue that they fixed now. Some issues remains, but mostly on MathJax's side (e.g., https://github.com/mathjax/MathJax/issues/2877).