Re: This confused me
Your signature is your acknowledgement that you have been notified so that even if you're not a crown servant or a contractor you are still subject to the act in the same way.
It's a law, it applies whether or not you sign a bit of paper saying you were notified. Being notified is largely a courtesy.
"Do you have to sign the Official Secrets Act to be bound by it?
It is not necessary for a person to have signed the Official Secrets Act in order to be bound by it. The 1989 Act states that a person can be “notified” that he or she is bound by it; and Government employees will usually be informed via their contract of employment if they must observe the Act."
https://commonslibrary.parliament.uk/research-briefings/cbp-7422/