It may help, in that it’s going to limit the amount of processing that each XML-RPC call is going to take – for instance, with the plugin enabled, they’ll receive an from the XML-RPC processor. While generating this error takes a little bit of processing, it is less that what’s required to pull a page/post from the database.
However, I believe that’s only going to be a small step and may not even show any real effectiveness in making your site available throughout the DDOS attack.
I would recommend further measures, like using a plugin which will rate limit the requests coming from any on endpoint (I believe that Wordfence does this – though don’t take this as an endorsement, as I haven’t specifically tested the configuration). A CDN and/or web application firewall may also help.
