No. Calyx isn't a fork of Graphene.
"CalyxOS draws heavily from the LineageOS community and code."
In my experience Calyx wasn't a good fit. Have had best luck with Graphene. Its reported to be more technical than Calyx OS but I don't find it to be a problem.
The issues most experience is when they try to load no free software and or FOSS that utilizes some sort of Google dependant.
About CalyxOS
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