None of those are messages/documents contain racist action by anyone at Coinbase. They are (1) a reaction to a blog post, (2) a reaction to a letter, and (3) a conclusion in a report. They are not concrete evidence of racism.
I'm not sure what your standard is, you said 'corroborating evidence'. This is evidence that corroborates what the employees told the NYT. Now you're at 'concrete evidence'. What would that be? That someone yelled racial slurs at employees and then handed out signed receipts? The coinbase 'prebuttal' doesn't concretely dispute much either, it says you're going to see some stuff in the NYT that's 'hard to read'. If anything, it's striking how little both pieces disagree about the employees' allegations.
For example, the article talks extensively about Layllen Sawyerr's case. So I'd expect corroboration to look something like "we talked to suchandsuch other people who confirmed she was treated unfairly", "we read this email in which she was treated unfairly", or perhaps "we heard about this specific personnel decision which was unfair to her". A lack of corroboration doesn't mean her accusations are false, but a lack of attempt to corroborate is very troubling from a national newspaper, especially when the subject of the article is going on the record to say the accusations aren't true. Either Milosevich and Coinbase are lying about their investigation or Sawyerr's story is false - isn't the NYT curious to figure out which one?
