51% doesn't tell you much by itself. Benchmarks like this are usually not graded on a curve and aren't calibrated so that 100% is the performance level of a qualified human. You could design a superhuman benchmark where 10% was the human level of performance.
Looking at https://www.tbench.ai/leaderboard/terminal-bench/2.0, I see that the current best score is 75%, meaning 51% is ⅔ SOTA.
This is interesting, TFA lists Opus at 59. Which is the same as Claude Code with opus on the page you linked here. But it has Droid agent with Opus scoring 69. Which means the CC harness harness loses Opus 10 points on this benchmark.
I'm reminded of https://swe-rebench.com/ where Opus actually does better without CC. (Roughly same score but half the cost!)