That means that at 170,000 someone rebuilt the engine. However, because everything else on the car still has 210,000 miles it actually doesn’t add as much value to a used car as you might think. Also, a rebuild can mean a lot of different things, so it’s generally not the same as having an engine with only 40k on it and without documentation of what was done you don’t really know at all. Also keep in mind that most cars these days don’t “die” from a worn out engine-- the other stuff wears out and you get sick of spending money on such an old car.
I’d generally put a rebuilt engine in the “plus” column, but it wouldn’t be a deal maker for me.