You're only calling choice(cat_sayings)
once, at the top level of your module, and never again. So, it's going to pick one random choice and never pick a new one.
To fix this, just move the code into the function:
def cat_job(sayings):
print choice(sayings)
# ...
s.add_cron_job(cat_job, args=[cat_sayings], second='*/30')