Not true. There's not much risk of someone pulling old data out of the internal storage of a reflashed tablet, but SD cards and the internal MoviNAND storage on the N810 are a different matter.

Just as with hard disk drives, etc., until the old data is overwritten, it is recoverable. Recovering photos is especially easy due to the wealth of software designed to save family vacations from accidental deletion. See the Google search results for "flash card data recovery".