Yes, that is the point of their question. That’s also why I mentioned that the two glass blocks together don’t have a visible edge in the middle, meaning it would solve their issue. I did however want to mention the problems that the glass material comes with, such as the surface details.
@kanapuro an option for you would be to export the water parts to Blender as an OBJ file and then joining them together in Blender. You can do this by pressing CTRL + J, once you’ve done so, go into edit mode by pressing tab and then delete all doubles by pressing ALT + M and selecting the merge by distance option. This should fix any bad modelling practices. After you’ve done that, I would check the water and see if you don’t have any faces on the inside. If you do have those, go into face select mode (by pressing 3 on your keyboard) and delete them.
Note: You could probably do this in ROBLOX in some way, but I wouldn’t know how to and I generally find Blender easier to work with in regards to unions and boolean operations.