The heads up is enough to read the message but not enough to decide if you need to act on it or not. Before you can process it the shaking starts.
In Mexico City the seismic alert gives like a minute (sensors are on the coast off Guerrero 350km away). Communities closer to the usual epicenter get almost no warning.
There are no tsunamis in Mexico City haha
You are welcome to think it is ridiculous for needing 5 seconds to process what alert is going off. But the simple reality is no good disaster plan should assume that everyone will immediately understand what is happening.