Cinebench (SSE2) test runs on that machine only for few seconds so maintaining 5GHz during that period wasn't that difficult. CPU basically didn't have enough time to warm up
I bet that 28 core @ 5GHz would not survive few hours of encoding in x265 (4k + preset veryslow + AVX2). That 5GHz would most likely drop to base level in couple of minutes
Whole system could even crash with forced AVX-512. Yes. So for me 5GHz on that CPU is fake. Extremely desperate move in reaction to Threadripper 32core.