Spoken intentions are notoriously unreliable. The problem with using words to convey information that must be believed, and with using vocal signals in general, is that they are too easily falsified. Thus to really convince a rival to stay away, a lion should use an assessment signal. After all, owner or not, any lion could proclaim himself one by roaring, and just because he roars doesn't mean that he really will attack an intruder. There are two ways that a conventional signal such as roaring can be made honest.
Lions seem to do both.
Members of a coalition often roar in group choruses in such a way that makes it obvious that they are roaring together and not in competition with each other. Male lions also call the bluff of any roaring would-be resident. If an intruder roars inside an owner's territory, he is confronted, escorted away, or attacked. I confirmed the generality of this through a series of 40-some playback experiments in which I broadcast strange male roars to residents within their territory. In almost every case, the owner aggressively approached the speaker, looked around for the intruder, and only then started roaring himself.