Deciding on an ISA like RV32I only sets constraints on your architecture, it doesn't give you any of it for free. So I think it's entirely valid to say that it's a custom architecture if they didn't use any pre-existing CPU design.
Basically, they had the API interface and implemented everything behind it. I think saying that "it's an RV32I core" vastly underrates the design work that goes into actually implementing an RV32I core from scratch.