The raw content for this specific post (and the one below) on your forum shows that it is an uploaded image, not an external link. Because of Client side image optimization enabled by default, you will be able to upload large images.
Your initial issue is about inserting an external image link.
That said, I’m unsure if Discourse behavior is expected not checking the image size when pasting an image link. 
Can you share the value of upload_size in your app.yml?