The Server class defined in index.d.ts does not seem to match the Pretender class in ./src/pretender.ts that it is supposed to represent. It is missing the handledRequests, passthroughRequests, and unhandledRequests properties. There may be more missing types, as I haven't examined every prop closely.
It seems to me that instead of having a separate type definition file that has to stay in sync with the class, it would be better to export the types from the class directly with rollup.
I have a PR ready to go that implements this fix. I'll post it in a few minutes and reference this issue.