Aha! I knew there was something else bugging me!
Their destination was "the Alpha Centauri cluster". Let us assume that they meant triple star system of Alpha Centauri A, B and C, which is our next-door neighbour, at a little over 4 light years away. They left Earth 400 years ago, giving a minimum speed of 1% of the speed of light, possibly as high as 10% of the speed of light, if they arrived 350 years ago (I don't remember, but that's the number people are throwing around). These speeds are high but not unreasonable, so I'm inclined to believe that they are at the closest star to Sol.
So where did the other aliens they'd encountered come from? Neptune is four light-hours out, and would have been passed very soon after take-off, and after that, you just have a handful of dirty snowballs, only a few of which are large enough to be noticeable. Is life so common that, even in the Oort Cloud we can find societies advanced enough (or perhaps individuals powerful enough) to be worth defending against?
Also, had they really been fighting Mutts for 350 years (assuming that's when they arrived in orbit) without noticing that their ships didn't seem to be capable of interstellar travel? Didn't anyone think it was odd that they controlled such a huge swathe of space, some 30 or 40 light years across? Didn't any ever try and reverse engineer the Mutt's FTL drives?