The author deliberately added the files to the repository. That’s (part of) the problem. .gitignore is about saying “I don’t want to add matching files to the repository, so stop telling me they’re not in the repository”.
The README transfers almost 100mb. If I'm on my phone with no wifi and I see an interesting repo as I'm a neovim user, I will just lose 100mb from my data plan with no warning. One doesn't expect that when visiting the README of a code repository.
Then what happens when they change something that makes the gifs obsolete? Will they add a commit with updated gifs that doubles the size of the repository? Most developers use
git clone $URL