I created a custom faction that has the trait synthetic life but the assembly construction is linked to the yor tech tree. The focus of my Race's tech tree was intended to be technology and military but if I choose to do that and keep the trait synthetic life then my entire race is limited to 10 population.
In short the synthetic trait is useless without the yor tech tree severely limiting the customisability of synthetic race. If the synthetic trait is going to prevent population growth organically (which it does successfully) then the ability to create population should also be tied to it (as opposed to a single tech tree).
Also as some extra information I just created a race that was not synthetic but used the yor tech tree. It doesn't have the assembly so I imagine it's working as intended there.