Anyone can claim a rank in a comment. Sign in through Steam and Dewarded gives you a page with the medal OpenDota reports for your account, so you can link to it instead of posting a screenshot.
Add that link to your Reddit profile. Anyone who doubts you can open your profile and see it points to your medal page. Steam proves which account is yours without showing us your password, and your page never shows your Steam account. You can delete everything at any time.
Valve doesn't publish MMR. This is the usual range for this medal, from community estimates on Liquipedia (last revised 2019). Real MMR is often a few hundred off.
