The way you read Rev. 19:17-18 comes from a failure to take other passages into account like Isa. 28:10 says you should. Zechariah tells us who those people are:
"And this shall be the plague with which the LORD will strike all the people who fought against Jerusalem: their flesh shall dissolve while they stand on their feet, their eyes shall dissolve in their sockets, and their tongues shall dissolve in their mouths. It shall come to pass in that day that a great panic from the LORD will be among them. Everyone will seize the hand of his neighbor, and raise his hand against his neighbor’s hand;" - Zech. 14:12-13
Jerusalem is not mentioned in Revelation 19
Read zechariah 12-14 and you will see that God speaks of two Jerusalem’s one the literal city and one the church. In zechariah 14 we see that Jerusalem the city is destroyed but Zechariah 12 shows us that Jerusalem is the immovable rock thus two Jerusalems. The literal city at the beginning of chapter 14 was destroyed back in 70AD when the day of the Lord came upon Jerusalem.
When did the living water flow from Jerusalem? When the Holy Spirit was sent out to the world through the deciples back in the first century. I do take other biblical passages to understand the bible.