The Jetstream benchmark is made by the WebKit team. Its scores will vary per browser, so you have to compare browser to browser. I compared an iPhone 11 running Safari to a MacBook Pro running Safari. Apples to apples.
Moreover, while the Threadripper 2990WX is a really awesome processor, single core benchmarks (I think Jetstream is mostly limited to a single thread) aren't particularly its strength. Over multiple runs it should beat your Macbook Pro, but not by a huge amount. If not, take a look at how you're cooling that beast :)
I updated a six core i7 3930k with a sixteen core 1950x threadripper on one of my boxes. Biggest reason for the update was the disk IO (m2 drives) and number of cores and I'd had the 3930k since the launch day at Microcenter. On Windows, single threaded at stock speeds, the cores were comparable from a 6-8 year gap between the two CPUs. For hosting virtual machines... the speed did not matter as much as having an abundance of physical cores. Still - I was not expecting the 'core speed' as reported by a video game to be as close as it was.
The Zen2 (3900x) core speed on the other workstation reported as almost twice as fast, with 12 cores. Really wish that TR4 board supported the 39xx threadripper series.
Moreover, while the Threadripper 2990WX is a really awesome processor, single core benchmarks (I think Jetstream is mostly limited to a single thread) aren't particularly its strength. Over multiple runs it should beat your Macbook Pro, but not by a huge amount. If not, take a look at how you're cooling that beast :)