Interesting they think it was written to after the dent. I'd think it'd error during write.

The floppy disc controller doesn't have a way of knowing if the write worked or not. There are some corner cases here, like the drive can signal a fault line if it's on fire, but if there's simply a dodgy patch of disc surface, nothing will be apparent during the write.

So, the operating system could verify the write by reading it back, but I don't think the BBC Micro disc ROMs typically did that for file writes -- only for formats.