No. Memcached on a reasonable server will do millions of requests per second. 50,000 updates per second is nothing for any modern cache.

Also, replication has nothing to do with whether or not "without a cache" is a meaningful statement. The point is that by holding their entire data set in RAM, they've nullified the need for a cache. Effectively, their database is their cache.

And considering the data isn't even written to disk for about 15 minutes, it's really more cache than database anyway.