In bash's man page clarity is sacrificed for brevity. getrlimit man page describes it in more detail:
Code:
RLIMIT_RTTIME (since Linux 2.6.25)
This is a limit (in microseconds) on the amount of CPU
time that a process scheduled under a real-time scheduling
policy may consume without making a blocking system call.
For the purpose of this limit, each time a process makes a
blocking system call, the count of its consumed CPU time
is reset to zero. The CPU time count is not reset if the
process continues trying to use the CPU but is preempted,
its time slice expires, or it calls sched_yield(2).
Upon reaching the soft limit, the process is sent a
SIGXCPU signal. If the process catches or ignores this
signal and continues consuming CPU time, then SIGXCPU will
be generated once each second until the hard limit is
reached, at which point the process is sent a SIGKILL
signal.
The intended use of this limit is to stop a runaway real-
time process from locking up the system.
For further details on real-time scheduling policies, see
sched(7)
In short, it doesn't do what you expect it to do, you should've used timeout (as in 'man timeout')
