The characters in the party are size Medium, the Black Pudding was size Large. Thats not 2 sizes larger or smaller.
The first opportunity they had to flee was the first moment they were actually winning. They tried to press their advantage rather than run away.
And also, watching the VOD now, when Salix and Silent did motion to leave (after the other 2 had been downed), the Black Puddings moved 20 feet and attacked them. I believe this was an error. They have 20ft of movement, they were in difficult terrain, they dont have a swim speed, they should only be able to attack and move 10ft.
The way they were moving and attacking in the difficult terrain - players without bonus dash would never be able to escape. They would always be in range of their movement.