What if an interviewer says “HR will get back to you”?

If someone really impresses at an interview and I am fairly confident that they will be invited back for the next round, then I will tell them that HR will get back to them (to tell them the good news).

It means that HR will get back to you.

If I am involved in a recruitment process that could involve 100+ applicants, I am absolutely not going to contact them all directly. If someone really impresses at an interview and I am fairly confident that they will be invited back for the next round, then I will tell them that HR will get back to them (to tell them the good news). If someone has really blown it, I will tell them that HR will get back to them (to tell them the bad news).

If I have decided that a particular person is exactly who I want to hire, I will tell them that HR will get back to them (with an offer letter).

It does not signal good news. It does not signal bad news. It really means nothing at all.