I fail to see how this email address does not qualify as common sense valid though. Maybe it’s just me being surrounded by tech-saavy people, but I have a bunch of friends using emails like i@domain.com and me@name.com.

Valid is a binary condition. Either an address is valid and you will be able to send mail to it, or it is not. That is usually the sole reason for validating an email address. If a bug in the validation logic causes an unusual email address to invalidate then the code is definitely wrong, not the user.

If the PM specifies a requirement to filter out email addresses with a single character before the @ sign, then that's a different thing. Maybe there is a business requirement to filter out people who own a domain and give themselves a single character email address? In that case it should be explicitly written into the validation logic and not done as a side effect of sloppy coding.