The one employer rule would not in itself make someone 'employed' rather than 'self-employed'. If the pilot is flying all his annual limit in terms of hours for Ryanair then he could not work for anyone else. In the current climate he might not even be able to find other work. That would not make him self employed.
There are many items affecting your contract that can determine your status and none of them are set in stone. Working for one client is only one of these. Others inclube being able to send someone else to work in your place without having to get approval from the client. I dont think they would be happy if you sent you brothe/sister/mother in one day to fly your sectors but that would not make you 'employed'.
The main thing they want to see is how you are treated differently from the full time employees. This may be in the form of extra fees you have had to pay that an employee hasn't. It might be you are not subject to staff appraisals. Even things like not being invited to a xmas staff party can help determine your status. Things like not being in a company pension scheme all definately help as well.
What can really get you caught as being employed is when the company pays your training. When i hear of Ryanair pilots having to buy uniform, pay for training, hotels etc. I have always assumed this was due to the above and what the individual has paid in expenses they have more than saved in taxes.