The C10k problem is 15-20 years old. Tcl uses select() which can handle a maximum of 1024 FDs. In other words, it would not be able to handle more than 1024 connections in parallel.

Moreover, Linux and FreeBSD have introduced syscalls comparable to select(): epoll (in 2002) and kqueue (in 2000). Those are way faster than select() and I think there's (virtually) no limit on FDs. Tcl must be using select() for its portability I guess.

So, handling a lot of concurrent connections is an old problem. In 2006, the C10k problem was way behind. Even at that time, I would have expected a program to be able to handle thousands of TPS.