It depends. In order to contribute to a solo 401k, a person must have earned income. If someone just owns multiple rental properties then they would not qualify, because rental properties are not considered earned income.
However, profits earned as a flipper or property manager is generally defined as earned income. Accordingly, this would qualify for contribution to a 401k plan.
We work with real estate clients to structure their operations to try to qualify for a solo 401k.