According to Apple's MacBook Pro marketing page [0], their SSD supports sequential read speeds of up to 3.2GB/s. So this isn't as far-fetched as you imply, even if we're discounting other factors like the filesystem cache.
In this case isn't the "already in RAM" test a more accurate reflection of performance anyway, as we are talking about the performance of grep and not the IO subsystem?
There are many cases where grep's performance won't be bottlenecked by IO, or at least not impacted directly by a bottleneck there. Anywhere when the input is coming from another program, essentially, and even if that task is IO bound it might be sending output to grep much faster than it is consuming input (perhaps gunzip <file | grep searchtext).
And in the case of searching a log file interactively, it is common that you won't just run grep of the file just once in a short space of time, instead doing it a couple of times as you refine your search, so for most of the runs it will be in cache (assuming the file is not to large).
Nearly ever SSD listed achieves well over 1GB/s in an actual benchmark, not just on a spec sheet. And these are just boring old off the shelf consumer drives. Nothing crazy.
So yeah maybe not over 400MB/s, but all of them are over 200MB/s. Sequential speeds really spiked as densities kept increasing.
Note that you're not going to get this with SATA SSDs, you need NVMe, it's a 5x difference in throughput and IOPS.