Yes, they knew about the relativistic effects. And they could obviously send some data. But somehow they can't figure out that they've been getting a repeat of the same ping for the last 10 years? And even if they couldn't figure that out, how could the person on the planet possibly send a ping every year, if she only spent an hour and half on it according to her frame of reference?
They expected a ping every year, if it ever stopped pinging they would write off that planet. It was up to the scientists to keep the ping going, if they considered the planet still a suitable candidate for humanity. The signal they received from the time dilation planet was supposedly "looped around the black hole" and repeated yearly.
If I remember correctly, one of the characters hand-waved it as (paraphrasing) "multiple echoes of the same signal". Meaning there were no meaningful timestamps
