It’s also worth pointing out that the Septuagint version of Ezekiel doesn’t mention 390 days. Instead, it refers to a total duration of 190 days in verses 5 and 9, comprising separate periods of 150 days for Israel (verse 4) and 40 days for Judah (verse 6). The ‘150 days for Israel’ in the Septuagint likely refers to the period from the exile of Israelites by Tiglath-Pileser III in 732 BCE until the exile in 582 BCE alluded to at Jeremiah 52:30.
The reference to ‘390 days’ was most likely introduced to the original text because there was never a notable return from Assyrian exile of the northern kingdom so it was edited to instead focus on the descendants of Israelites who avoided Assyrian exile and fled to Judah in the 8th century BCE, whose descendants were later exiled to Babylon with the Jews.
Ezekiel 37:16 identifies those Israelites among the people of Judah at the time of the exile, distinguishing between ‘Judah and the people of Israel who are with him’ from ‘Joseph and the house of Israel who are with him’.