The Linux kernel treats threads no different than processes that share an address space which is unusual in UNIX land.
They also share a signal handler table, open file table and current working directory. If a process-directed signal is sent to the process any of the threads with the signal unblocked can handle it. If a default signal disposition causes one thread to exit (eg SIGKILL or SIGSEGV) then all threads in the process exit. The exit_group(2) syscall causes all threads to exit (which is used to implement the POSIX exit() libc function).
Really, "threads mostly are just processes" hasn't been true for a very long time. Mostly the only ways in which that is still true is in scheduling, where Linux schedules each thread independently (what POSIX calls PTHREAD_SCOPE_SYSTEM) and in credentials, where threads each have their own set of current, real and effective uids / gids (and glibc has to use a signal-based hack to synchronise them).
The point about the quote was when Solaris and BSDs were all doing special treatment of threads (LWPs/LKWT) for M:N threading, Linux since day one started with treating threads just like processes. The changes to special treat threads to solve problems (clean up manager threads hackery, signal handling improvments, clone syscall improvements, PID semantics etc.) came in later with NPTL as you said but by that time Solaris 9 had already adopted a 1:1 threading model.
They also share a signal handler table, open file table and current working directory. If a process-directed signal is sent to the process any of the threads with the signal unblocked can handle it. If a default signal disposition causes one thread to exit (eg SIGKILL or SIGSEGV) then all threads in the process exit. The exit_group(2) syscall causes all threads to exit (which is used to implement the POSIX exit() libc function).
Really, "threads mostly are just processes" hasn't been true for a very long time. Mostly the only ways in which that is still true is in scheduling, where Linux schedules each thread independently (what POSIX calls PTHREAD_SCOPE_SYSTEM) and in credentials, where threads each have their own set of current, real and effective uids / gids (and glibc has to use a signal-based hack to synchronise them).