Aliases were used in the infamous hush agreement involving Donald Trump and Stephanie Clifford. The alias for Donald Trump was David Denison and the alias for Stephanie Clifford was Peggy Peterson. Did the document state who the aliases stood for? How could the agreement be enforced as an agreement between Trump and Clifford if it seems the agreement involved neither of them?
A contract is an agreement between 2 or more people - who cares what they call themselves?
If someone asserts that the person named in the contract is you and you assert that it isn’t then they would need to provide sufficient evidence to prove it was you on the balance of probabilities. It the contract has your name on it rather than an alias this is obviously easier.