Discord messages aren't actually unlimited; they are limited to 2,000 characters, unless this restriction was recently removed. I have personally encountered this, mainly when using code blocks.
Can you tinyurl a url with more than a few thousand characters?
Even if this were possible (which, even if tinyurl supported it, would break most browsers) you would be effectively hosting your content on tinyurl.com. Again, this is no different from hosting the file on any of a zillion public data hosting services.