Registering systematic review in Prospero
First of all, If you don't think it's mandatory, and you don't want to do it, just say it's intentionnal.

Since, as you said in comments, this database is apparently used when you plan to undertake a SR, it would have no value for you to register in it, since you are already in SR.

But you may ask the reviewer why he got the urge for saying that , and if he thinks it would be interesting for you to register on it anyway.