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Most clay tablets pulled out of the ground are unfired. The act of removing unfired tablets that have been in the ground for so long kicks off some decay, so it used to be common to fire excavated tablets to preserve them. There's modern conservation techniques that remove the need to fire tablets, but those techniques post-date the lawful flow of material culture to western museums, so pretty much everything you'll see is fired. While fires in antiquity might have inadvertantly helped preserve some tablets that might not have otherwise survived, this is hardly the most common case. The talk of fires preserving tablets is mostly used to illustrate the stark difference in durability between records recorded on clay and records recorded on vellum or papyrus.