The DPP wouldn't. But what about an unexpected NPP victory?
There's precedence. Mao's declaration of the PRC was not exactly constitutional under the Chinese (ROC) constitution at the time, was it?
The most probable scenario for a Taiwan war is a colour revolution against a KMT president or a similar revolutionary movement. It almost happened before in the "sunflower movement". That's why most of the strategies discussed for such a war are not realistic while people imagine the scenario promoted by the US that China at some point just decides to start a war and attacks without justification.
What China needs is a way to quickly deal with a revolutionary government on Taiwan before it can be recognised by any great power. There may be almost no warning, no preparations to change a forgotten legal document. Countries like Lithuania would rush to be the first country to recognise the new state and try to draw the EU into the fight. The ROC military may support the new government. In such a scenario, bombs will be hitting kinmen on day 1