Surface rust is fine, in fact, most rust you can see is fine if DRY, it’s when you let it sit in damp/water that it will progress. I’d be impressed if you ever managed to rust through a bike from the outside inwards, I’m sure it happens, but I’ve never seen it, even on garden bikes left out 24/7, it just doesn’t see the conditions necessary as it will always dry out on the outside.
But,you can easily rust through a thin steel frame tube from the inside out if the moisture gets in and can’t escape, but as long as it can get out again it’ll likely be fine.
If you want an example of just how crappy something can look and still be fine have a look through this thread:
http://www.retrobike.co.uk/forum/viewtopic.php?t=192838
But then I’ve seen a 90’s Dawes Galaxy down at our workshop that looked pristine, until we started poking it and fingers went through the paint and into the tube 😯