He's correct in the sense of trying to catch SQL injection via input validation - that's a losing game. He isn't saying "don't validate your data at all", that's a different issue. Ultimately, your OWASP issues (XSS, SQL Injection) related to input are going to be prevented by appropriate escaping and data handling across your entire stack by default (key word is default, "trust the devs" is not the right answer). Input validation isn't the ticket.

This is a prime example of the Robustness Principle. https://en.wikipedia.org/wiki/Robustness_principle

Would you be so kind to explain me the attack vector if the user input is never possibly treated as part of the code?

What I came up with is this: user name is stored in the database, and some new junior developer in a large team reads it in the backend code, and immediately plugs into another SQL query using string concatenation. BOOM!

But on the other hand, the very same junior developer can forget to sanitize the inputs before storing them (or do it incorrectly), so there.