“porcupine” strategy, is not as you state.
Rather - a porcupine — a prickly, stubborn nation that would be hard for an invading Russian army to digest
So more about preventing Russia from absorbing Ukraine.
You are 100% correct as to what the “porcupine” concept is supposed to be. My view is that the Biden administration invoked the porcupine rhetoric cynically to justify massive arms and aid transfers to Ukraine (we have to give peace a chance...), with no serious expectation of “prevention”. Ukraine is not Switzerland, and there was a hot civil war underway, with Luhansk and Donetsk recognized as independent states by Russia.
Biden’s blustering made invasion more likely (if that was possible), and there was no real diplomatic solution is the offing. Given Putin’s personal situation, Russia’s security and economic imperatives, Putin’s public statements and prior actions in Georgia and Crimea, his Eurasian Economic Union project, etc., could anyone believe Putin would be deterred by untested threats? Biden’s comments such as “as long as it takes and as much as it takes” suggest a long, ugly digestion process. If whoever is running US policy expected prevention, God help us.