Can someone explain? I can barely parse this quote. Who are the 1P devs? Why does it matter that they didn't notice they had made a minor change? Why were they "tricked"?

Edit: I think I might understand it more now: This person got tricked into carefully reviewing the "entire" code, instead of focusing on the one minor change that was made to it, because they didn't realize it was only the minor change they had to review. In their careful review of the code, they uncovered vulnerabilities which were actually related to the original code (ie. VSCode) rather than the changes that this person was asked to review. Did I get it? I'm still confused about the use of "1P" here though.