Traditionally, many hard science fiction authors have been scientists. David Brin and Gregory Benford - authors of the hard science fiction classic Heart of the Comet, are both astrophysicists. Larry Niven and Greg Egan - two fairly prominent voices - both studied mathematics up to at least the graduate level. Sir Arthur C. Clarke, while not a practicing scientist professionally, worked on scientific journals, wrote technical material, and had connections like you would not believe. Stephen Baxter has a doctorate in engineering and taught math and science prior to becoming a writer full time. I could go on.
In any case, writing hard science fiction does not necessarily mean getting the nitty-gritty details of scientific or engineering processes correct, unless one is writing something super-technical like The Martian, it means adhering effectively to principles. The level of understanding necessary to write about established material in a given field for dissemination to the public is much lower than that necessary to actively pursue research to advance that field. This isn't even a specifically scientific thing, Tom Clancy became one of the most famous authors in America for writing about espionage and the military industrial complex in a way the was sufficiently accurate that he ended up meeting with actual military officials despite having a very limited military background. This sort of thing is very common.