I've received a "Notice of DMCA removal from Google Search" for a page was inaccessible to google as per robots.txt. Why did Google have this page in search if it was disallowed from visiting it at all?
Somebody should have reported to Google about this content. That is the only way you should have received a DMCA take down notice.
You stated that you disallowed the page in "robots.txt". Did you noindex, nofollow the page as well? If you did not do that, Google can crawl the page if its found on any external links pointing to this page.