Regarding the Disable XML_RPC plugin, as the plugin author, I did not realize that it was being recommended for this use by Network Solutions. I have also found a couple of other articles relating to the DDoS attack that mention it as a way to thwart the attack.

The plugin was not designed for the purpose of disabling pingbacks. It does one very simple thing - set the "xmlrpc_enabled" filter to false. This filter was introduced in WordPress 3.5, when the UI option to disable XML-RPC was removed and it was enabled by default. I had some edge cases where a site owner did not need or want remote publishing and wanted XML-RPC disabled to lessen his security footprint. I decided to release the plugin to the repository, in case anyone else found it useful.

I regret that Network Solutions and others publicized it to do something that it doesn't. I can only assume that it was not fully tested for this use case before that e-mail was sent.