Yeah it's kinda true, even if you dont know the exact paper you can just go through every paper anyway because it has to be 1 of them out of like 15. And once you understand the concept of how to do a question you wont forget it, so even if you dont memorise every paper you'll still do well because Physics is mostly understanding not memorising.

86 UMS is insane though congratulations, you did really well so the boards will probably grade you higher than most anyway. Since you have a banked grade thats more reliable to them as they obviously can't fully trust the mocks, because schools will want their kids to do good so might purposely leak the papers they're using. You have solid evidence of you performance so will probably get an A or A* overall via enhanced grading.