No it is not a typo, the regions are paired as old terminology if you think Primary and Secondary Datacenters or Active - Active DCs, so if there is a disaster in one site the other site will be takeover and it is called "Failover" process. As this process is all automated and behind the scenes of the live environment it is termed as Failover instead of Fall-back as the meaning if you see fall-back is something that is alternate to take care of. So in short there is no typo and it is correct.
Hope this helps.
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