Just like Strange could smell it, Doom's fear was reeking off him throughout the whole issue the moment he knew Reed was there. He was posturing yes...because he's afraid. You posture and preen because you're afraid people know you're afraid.
Now possibly, there is a survival issue here. I still think a lot of Doom's power is faith-based. The moment Doom starts doubting himself because Reed's there, his entire control might start break, Battleworld would probably literally start to break. The worse thing is when God stops believing in himself, that's pretty much kaput as far as faith-based powers go.
Have to agree, as far as the cosmic powers go, the Phoenix force was never up there with the Infinity Gauntlet level of cosmic entities (by that, I mean the ones they called in when Thanos had it were beings like Master Order and Lord Chaos; Mistress Hate and Lady Love; Eternity etc.)