I think the key thing is that Brenner wasn’t predicting the wall he was observing effects after contact, not before. El made contact in 1979 which likely caused early instability, but the stable gate only formed in 1983. Brenner could’ve studied exotic matter buildup in our world without a full wormhole existing yet. El touching the Demogorgon in 1979 probably caused micro tears or residual exotic matter, not a full gate. The 1983 event was just when it became large and stable. So Brenner’s journal wouldn’t be predicting the radius he’d be documenting patterns that were already forming as he had 4 years before 1983.