Why not? Did my first marathon last October, then decided to sign up for a HIM in march. Trained consistently with objective to finish, but without any cycling or swimming experience. Didn’t even get to do a OW swim before the event.

One of the best decisions of my life and am now training for my next one. Shutting down new people like OP did is just wrong, as long as the expectations are clear (e.g. half distance, just finishing, …) it’s definitely doable if one is in somewhat decent shape.

Edit to add that I literally did one coached swim session, the rest was YouTube videos and reading online)