With this much rye I’d be very surprised if it puffed up tall. Eric baked it in a deep clay baker which would have encouraged a little better rise. If you’re baking it on a stone or oven tray then expect a flatter loaf. I disagree… If you had baked this in a loaf pan (but I understand your reluctance) this is salvageable when it comes to getting a tall loaf however I don’t think a flatter loaf is necessarily a failed loaf. If its flat but has a nice crumb and its tasty then its a winner. So save judgement for when you cut into it and had your first taste.
Now even if you think you have the exact same ingredients you might not. I watched the video and Eric said rye flour and white flour. He ground his own rye berries but if he had a fine grind and you’ve bought a coarser grind then yours will need less water. The recipe doesn’t say what type of white flour. How strong is his bread flour and how strong is yours? Since you’re using 50% rye you have to be going for a very strong bread flour to expect any sort of good rise. 13% + but ideally 14-15% protein. What are you using?
P.s. ignore anyone who says things like “this is so easy”. You don’t know how many flops they’ve had before they started churning out “good” loaves. As a matter of interest how much have you baked with sourdough and rye?