Needles don't live forever and these have had a long and useful life.
I don't know for sure, but in most cases a "lifetime guarantee" is a guarantee against manufacturing defects, not wear-and-tear from normal use.
But the Denise people have a reputation for great customer service. If they don't actually replace or refurbish the worn-out tips, you can buy replacements for a few dollars. Weigh the possible cost against the time it would take to try refinishing them yourself, and the chance that varnish or nail polish or whatever would end up flaking into your yarn or making the problem worse.