You should be able to take constructive criticism of your work regardless if it's a side project or not.
Almost all of the side projects would have undocumented constraints and requirements on which the design/engineering decisions were based. And I doubt the interviewer would take time to ask for design constraints beforehand and then go through the code enough to understand its design/architecture and have interesting criticisms that would bring something positive for the side project in the future.
You can't criticize design decisions without context. I would not be welcoming to someone criticizing my side projects without first asking for context/design constraints the work was based on.
I would not lash out or whatever. But I would not take the criticism without some sense of other person's care for the side project in question.
Because they'll then try to make every part of the code perfect and with complete documentation for everything, probably spending more time trying to present it rather than working on it.
Yes, you basically answered the question your self. Tell the interviewer that you wrote that code ten years ago and while it works it's less then ideal, then explain why its not ideal and what a better solution would be.