Page 404s without .html

I’m working on a friends site because his URLs append .html and he would like all mydomain.com/page.html redirected to mydomain.com/page. I’m only a Jr web developer but I hopped into his .htaccess and set it as:

RewriteCond %{THE_REQUEST} \.html RewriteRule ^(.*)\.html$ /$1 [R=301,L]

Based on this SE post.

That did not work but it did enlighten me to a problem.

On some pages if you manually change the URL from /page.html to /page it throws a 404 at me. What I can’t figure out is why removing .html would send a 404 error.

Aren’t /page & /page.html the same page?