Do I need to fly along this spiked tunnel to get the secret exit?
So, while solving level 4-1, I passed the following sign.
Having flown below the secret I can see red "exit" blocks up there, which I know from previous levels will unlock a second path in the main world.
I can't get up that passage, so either I've missed something, or the dev's are lying to me.
Which is it, and how do I get the secret exit that's mentioned here?
Yes you do need to go through that tunnel to get the secret.
The key to getting it, is starting the level, and then tapping UPARROW once, and then leaving the game to do it's own thing.