I'm afraid not, that actually means that you shouldn't have parked there. They are providing free parking for two hours to allow you to shop in their store. The sign acts as a contract between you and the supermarket because you are parking on private land and if you break the terms of the contract then they have the right to issue a penalty charge notice (PCN).

However, you may be able to appeal the fine if it wasn't issued to you at the time and was sent to you in the post more than 14 days later than when you parked there. You could also try appealing if you only overstayed by 5-10 minutes as many car park operators allow a grace period (in fact, if you had parked in a council car park they legally have to give you a 10-minute grace period). However, you might be asked to provide your shopping receipt.