The story
According to Ernest Nègre in his book Toponymie générale de la France (1990), the French placename Sigournais comes from Old French Segurniaci, which itself has been derived from Latin Secorinus + -acum. So it appears that the resemblance to Germanic dithematic names starting with *segaz is superficial.
Etymology from Wiktionary, CC BY-SA 4.0.