I looked over my recent comments and did not find anything that seemed objectionable to me - but that's the problem in a nutshell, isn't it?
In this case yes, but not always. Ideally, when you looked back over what you'd written you would have found one or more things that on reflection you realise could be hurtful, even though of course you never intended them that way when you wrote them. That's the hope, when bringing something like this to your attention without specifics. But it hasn't worked out that way.
I will assume you're bound to respect the anonymity of the complaint, and hence you cannot be told which specific comment caused hurt. Granted, not all answers here agree with that anonymity, but this is not a criminal trial, or even an internal disciplinary procedure, and so it's up to the employer whether to grant it. I think your best move is to do what you can, and be seen to do what you can, within that system. Don't just say, "I won't improve unless you tell me who has accused me", that makes you look unwilling to improve, with a side-order of suspicion that you intend to be vindictive. It might also appear unpleasantly ironic to your boss, that your response to a criticism that your criticisms are unintentionally hurtful, is to demand to treat that criticism like a criminal accusation!
So assuming you do anything about it at all, I think you have two good options here:
Try not to say anything hurtful in future. You will not achieve the improvement that you would have achieved if your attention was drawn to specific things you do that are hurtful but which even on reflection you aren't able to see are hurtful. But you can go out of your way to be gentle. There's a fair chance this is what your boss expects.
Go back to your boss and say, "I'm sorry, I still can't see what I'm doing wrong here, so I'd really appreciate some help improving. Is there someone who can advise me on the way I write in general, without referencing the particular comment that was complained about? Are there comments other than the one that was complained about, that you can use to illustrate the problem I need to address?". Of course you can also do this without going back to your boss, if you have any kind of mentor, or just pick a colleague you think might be able to see what you can't.
It's also possible that your boss basically disagrees with the complaint, and doesn't expect you to do anything about it, but has to go through the motions. Which is rather unpleasant for you, since you now have a complaint against you and no means to remedy it or defend yourself. So if he thinks he's doing you a favour by letting it slide then he's probably wrong, and he's probably going to see the whole business repeated in future.
Far better, if he disagrees with the complaint, would be for him to go back to the complainer and say, "I'm sorry that you're hurt, but this is entirely in line with the comments that we expect to be given during code review, so we'll help you to respond to them in the spirit they were intended, which was to criticise the line of code on the page and not you. This is the result of the standards we've set for the whole team/company, not anything that aednichols in particular has done wrong, and so please do not hold it against him or conclude that because of what he wrote he must think badly of you". And never tell you that anything happened.
Finally, you can't really "make things right" with the person who was hurt. They seem to have raised the issue under condition of anonymity, and so they do not expect an apology from you. You could perhaps ask your manager to make some statement to them on your behalf, that you intended no hurt and are sorry to have caused it. Since you don't know what you did wrong, this will be very vague and so probably isn't worth doing, but your or your boss might juge that it would help. A statement like that, which your boss vouches for as sincere, might even cause the other party to drop their anonymity so that you can all deal with the issue in full. Long shot but you never know your luck.