On Reddit the normal person can be forgiven for not knowing the full context of quote, but this quote has come up on HN many times over many years.
I've never seen an "explanation." It seems self-explanatory. I haven't seen an apology either, but this was in the New Yorker:
When I asked Zuckerberg about the IMs that have already been published online, and that I have also obtained and confirmed, he said that he “absolutely” regretted them ... Zuckerberg’s sophomoric former self, he insists, shouldn’t define who he is now.