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... quo vulgi opinio est, tamquam mutationem regis portendat. igitur,...

In the Annals, Tacitus uses "tamquam," not to introduce an unreal comparison, but virtual o. o., meaning "on the ground that," "to the effect that." So also "quasi." Here, the meaning is that the "common belief" about the comet is/was that "it portends a change in ruler."