Pulumi is declarative. The procedural code (Python, Go, etc) generates the declaration of the desired state, which Pulumi then effects on the providers.
HCL is also not pure declarative code either. It can invoke non-declarative functions and can do loops based on environment variables, so in that sense there is really no difference between Pulumi and Terraform. The only real difference is that HCL is a terrible language compared to say Python.
I'm actually fairly sure HCL is Turing complete, it has loops and variables. But even if it is not all the way turing complete it's pretty close.
Pulumi may be declarative, but you use imperative languages to define your end state. The language you're actually writing your Pulumi in is what's most relevant to the point I'm making about maintainability. HCL isn't turing comlete, but even if it was, the point is that doing the types of things you can do in Python or other "real" languages is a major pain in HCL which effectively discourages you from doing that. I'm arguing that is actually a good thing for maintainability.
HCL is also not pure declarative code either. It can invoke non-declarative functions and can do loops based on environment variables, so in that sense there is really no difference between Pulumi and Terraform. The only real difference is that HCL is a terrible language compared to say Python.
I'm actually fairly sure HCL is Turing complete, it has loops and variables. But even if it is not all the way turing complete it's pretty close.