Also, follow-up question... how is this exactly a threat?

Did you read Herodotus? Herodotus 8.62 answers this question. 8.62 has Themistocles reply to Adeimantus' taunts that the Athenians are a people without a city by saying that the fleet is Athens and that they are therefore stronger than the other Greeks. He turns to Eurybiades and threatens to evacuate the Athenians to Siris, where Herodotus has Themistocles claim a prophecy told the Athenians to found a colony. Such a mass evacuation would not, probably, have been especially difficult. Athens had already evacuated its population when the Persians entered Attica, and as Themistocles mentions to Adeimantus they had a 200-ship fleet standing by. The Athenians would not have been the first to relocate an entire city elsewhere. Other cities had done so, by force or willingly under pressure. A few years after Salamis Hieron tyrant of Syracuse evicted the Naxian population of Catana south of Mt. Aetna to Leontini, and refounded Catana as Aetna, resettling it with Dorians. During the Ionian Revolt the Phocaeans abandoned their city, which is also in Herodotus, and sailed with their whole population first to Chios and then to Alalia in Corsica, where they likewise had been told by an oracle to found a city.

For the benefit of /u/79c4a06fbba64629867a, the threat that Themistocles makes in Herodotus is very real. The Peloponnesians, and especially the Corinthians, wanted to fortify the Isthmus and withdraw from Salamis, fighting the Persian fleet in waters around the Peloponnese. This would have been a tacit acknowledgment of the impossibility of retaking Attica and would have placed Athens in a subordinate position, strategically less important than the Peloponnese. Herodotus repeatedly has ethnic tensions between the Dorians and Ionians flare up, especially between Corinth (whose general Adeimantus is repeatedly a stick in the mud) and Athens. At several points Themistocles runs up against these arguments and resorts to several different kinds of trickery to force the other allies' hands. Previously he has used bribery, to get the other allies to fight at Artemisium (himself having been bribed by the Euboeans), and later he will send a secret communication to the king. The emphasis is simultaneously on the disunity among the allies and the necessity of unified action. If the Athenians had left, Herodotus' Themistocles reminds his Adeimantus, the allies would have had no chance. Athenians made up a significant part of the army (and the most experienced part against the Persians, having defeated them already at Marathon) and Athenian ships made up half the fleet. Had Athens simply left or negotiated with the Persians the Greeks of the Peloponnese would have been doomed