Signal supports anonymous connections, anonymous user accounts, secure operating systems, etc — not saying it’s easy, but neither is securely using SecureDrop to my knowledge. OpSec is hard by definition, if it was easy and cheap, counter measures including laws banning such activities would rapidly appear.
I agree SecureDrop is complex (in fact I mentioned that in another comment). It's way more complex than sending a file with Signal or other modern privacy apps, but at least all parts are private and secure (as far as we know) so it's up to the user to not make mistakes.
With Signal we know that Signal itself may be compelled to provide information about its users and I think that's why they wouldn't recommend their application for such use cases.
Again, Signal supports anonymous connections, anonymous user accounts, secure operating systems, etc. If anything, it’s more secure than SecureDrop because it doesn’t require using Tor, which is a huge red flag and appear to acknowledge odds of it being secure are questionable at best. Also, assuming user uses an anonymous connections, anonymous user account, secure operating systems, etc — Signal would have no information it could be compelled to provide.