An oversimplification.

Rice's Theorem states that all non-trivial semantic properties of programs are undecidable. However:

1⃣ It applies to the general case of determining ALL properties of programs.

2⃣ It doesn't preclude proving SPECIFIC properties about SPECIFIC programs.

3⃣ Smart contracts are finite-state programs with bounded execution.

While we can't prove ALL properties, we can prove MANY important security properties.

The correct interpretation is: "It's impossible to create an automated system that can verify ALL security properties of ALL smart contracts." This is very different from saying secure smart contracts are impossible.