upacārataḥ ≈ upacārāt, "periphrastic ablative", Whitney, Grammar, § 1098.
The GRETIL text:
utpannaḥ procyate vidvan nitya evopacārataḥ // ViP_1,3.4 //
I suggest something like:
"O wise one, the truly eternal one is said, by figure of speech, to be created."
Taylor 2021 is to the point in translating "by convention".