Note: This will not provide backend validation, just frontend. So technically, if you conditionally add the ...
I used this add on with 2.9.2 a few days ago and it doesn't have this problem. Which makes me think you have installed it incorrectly
If you downloaded it from Github then you should rename the containing folder from rsanchez-entry_type-0a0e58e to simply "entry_type".
That folder should then be placed inside system/expressionengine/third_party