Because, serializing db access through a single process only becomes a problem when the number of reads/writes get so large, that the process becomes a bottlenec.
And judging by the test the author of the linked article did, that would have to be a HUGE number.
That's only a theoretical limitation. 99% of all your typical insert / update / delete operations finish in the single digits of milliseconds, making the serial nature of SQLite writes a problem when you get north of 5000+ requests per second.
