Not at all, there is nothing physically impossible about someone engaging on projects that take centuries or millennia to complete, it just requires a lot of patience and effort. Finding or even attacking an alien species does not fundamentally require anything disallowed by physics, it just requires a long timescale to do slower than light. My assumption was just that any hostile aliens would simply conduct those hostilities over very long periods of time. Having interstellar travel at all, assuming no ftl, sort of implies a willingness to undergo these kinds of long term efforts anyway, and it doesn’t seem absurd to imagine that anyone with the technology to have those kinds of energies at their disposal might also have very advanced medical technology, such that they might live a very long time, which could lend itself to more long term thinking.