As Roberto says, the wording of the email is confusing!!
However, I've recently left O2 & I used my MAC within a couple of days of receiving it. This was based on O2 verbally confirming that the 7-days applies between MAC issue & date of changeover (NOT giving the MAC to new ISP).
My new ISP (BT) logged it into the system immediately & I received an automatic response from O2 acknowledging that I was leaving. Because the actual changeover date was later than 7 days after MAC issue, I was NOT penalised! In other words, this confirmed what I had been told!