The book is an exploration of and presents an argument for militarism. That alone doesn’t make it propaganda. While many of the sentiments, implications, premises in the book carry a clear bias, the book nevertheless invites the reader to engage with and reflect on the ideology rather than aiming to manipulate and indoctrinate the reader.

I’d say the earnest argument presented by Heinlein in ST is flawed and morally objectionable, but not a piece of propaganda.