The problem seems to be an issue in the org source. The relevant function, `org-format-latex`, contains a line

```
(let* ((math-regexp "\\$\\|\\\\[([]\\|^[ \t]*\\\\begin{[A-Za-z0-9*]+}")
```

which indicates that the latex regex is hard-coded in this case. In other words `:matchers` does not work right now. It ought to be fixed with the next major update to org's latex handling: https://list.orgmode.org/orgmode/[email protected]/.