Sorry, wish I could give you sound advice. If it's a good company and they are offering to fix it, free, you may want to take it.

It's hard to tell why it's pitting. I'm not a plaster/concrete pool guy--mine is vinyl. But I wonder about how you let the replastering cure, and what your chemistry has been since they did the new plaster. Again, I'm not the expert but I've seen that the curing period of new plaster is critical to its length of life and must be done correctly.