How to flood fill empty spaces between Rooms?

Greetings,

I am trying to recreate the following: Rooms and Mazes: A Procedural Dungeon Generator – journal.stuffwithstuff.com .
I am able to instantiate rooms and spread them up so that none of them are touching, meaning there is empty space between them. Now I want to generate a maze in that specific empty space. He uses flood fill to find the empty space, but I have no clue how to use it.

If anyone can shed any light on this please let me know,

Thank you

Floodfill is an algorithm used for you guessed it, filling in space
It starts out by placing down the starter tile, then every turn you fill in every tile that is:
a.)not a wall
b.)reachable from other tiles that you painted already

here you can find pseudocode: