Not true, for the OP's post anyway.
People can enter into their own agreements, but as soon as someone takes it to court the agreement is close to worthless unless it is done correctly.
For a binding financial agreement to later be recognised by a court you both need independent legal advice, and even then it can be struck down in certain circumstances. You both cannot use the same lawyer, and you each should find your own lawyer. Do not even suggest a lawyer to the other party.