The flag "in need of moderator intervention" is sometimes the only option in the Android.
Example below, flagging the (now deleted) answer: https://superuser.com/a/1305357/116475
Typically there should be more for flag options:
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(now deleted) answer
That's the key. The answer got deleted in the interval between the page load and your flag attempt. The first stage of flagging is getting available flag options from the server. The server sends them based on the current post status, and the only way to flag deleted posts is for moderator attention.
This isn't specific to the app: the main site works the same way, although the site is more likely to update the post status via websockets.