Well as soon as she arrives, she says “we haven’t much time” and they plan an attack centered around the Death Star coming to them…
The line in the movie is ‘we have no time for our sorrows, commander’. Which would be true whether or not the station was on its way, since they would be keen to avoid another loss like Alderaan.
From the Revised Fourth Draft:
LEIA
We don’t have time for our sorrows,
Commander. The battle station has
surely tracked us here.
(looking pointedly to
Han)
It’s the only explanation for the
ease of our escape. You must use the
information in this R2 unit to plan
the attack. It is our only hope.
So even then she is only making an assumption.