Hi there,
A 200 is a rewrite or a proxy rule (cf Redirects and rewrites | Netlify Docs), which if used in that context does the opposite of what you want - it shows your site (mydomain.com) content at the netlify hostname (https://yoursite.netlify.com), instead of telling the browser “no, this is not the canonical URL, instead go over to https://mydomain.com” which is what a 301 redirect would do.
Those docs with the 200 example are buggy and have been live for a year and a half and you’re the first person to notice 
! Thank you for calling it out; I’m fixing it now.
Doesn’t matter if it goes in _redirects or netlify.toml; both are parsed after deploy and applied to your site.