I'm not in a position to say if that's true or not. It happened, 950 years ago, and we live with the results today. I'm quite sure that every event in history has both good and bad results. When looking at history, though, I like to look for the changes.
In this post I wrote about how English is currently changing. In this article the author, while bemoaning the Norman victory, discusses how the language changed after 1066:
Englishness became, almost by definition, a badge of subjugation. Human nature being what it is, people soon began to adopt the names and manners of their overlords. On one English farm in 1114, records Peter Ackroyd, the workers were listed as being called Soen, Rainald, Ailwin, Lemar, Godwin, Ordric, Alric, Saroi, Ulviet and Ulfac. By the end of the century all those names had disappeared.Ch-ch-ch-changes.
The status of the defeated English is often illustrated with reference to the vocabulary of meat. The Anglophone farmer in the field used plain Saxon words for his livestock: cow, pig, sheep. But by the time these animals found their way onto his Norman master’s plate, they had acquired French-derived names: beef, pork, mutton.
More telling, though, is the political vocabulary introduced under the Normans. Out go witan, folkmoot and folkright. In come fealty and homage, fief and vassal, villein and serf.