Right, I agree. Though, for reference, a lambda function isn’t hosted as part of a site. It has its own completely separate environment that it runs on. Each function is completely separate from other functions. Considering that, you can see how it’s not simple to share code between functions.
That said, the link I shared where someone made their utility functions into npm packages, they are not actually publishing those functions to the npm registry. It is a local package, whose scope is specific to that particular repo.
Also a correction: The link I share was not to the correct post. This is what I meant to share: Using require to include a relative module in Netlify Functions on Node - #18 by nomadoda. The post also share their repo which shows what they had done to accomplish the solution/workaround.