The King that Crawls

[Paul Farquhar]

Tiamat was a regional Untheric deity even in 1e - https://forgottenrealms.fandom.com/wiki/Untheric_pantheon

Not as detailed in the Monster Manual - as previously mentioned she was first named as a (optional) deity in Deities and Demigods.

As for the source you cite, you might want to check the date of the primary source - 1990 would make it 2nd edition.