I agree to an extent. If I'm offered standard market rate then this will be very different to a situation where a developer has factored into the price the assignment of all IP. I agree that in your case, the developer would be well positioned to argue that an attempt by the employer to enforce should be thrown out as a restraint on trade (at least under UK law).

On the second point, the contract isn't unenforceable as a consequence of one party fraudulently/mistakenly attempting to give away something they are not in a position to.

If I'm the company, I take an indemnity from the developer which means I will be covered (well subject to the insurance/assets of the developer) if a previous employer attempts to assert its rights over IP I'm using. In the same way as it's the employee's decision to assign all rights, it's the company's decision to accept the assurances of the employee that the employee is in a position to assign.