I agree fully with this statement, except for one important omission. This is that flashbacks are used as an escape mechanism, but they are equally, if not more, used as an escape mechanism for Willy to escape his own failure as well as the failure of his sons. Of course, the two are very closely intertwined, as part of the failure of the sons is the way that they have been brought up by Willy. This theme is of course highlighted with Bernard's success as a lawyer compared to Bif and Happy's lives and what they have achieved.
However, you are right to indicate that Biff and Happy trigger off these flashbacks, and increasingly Willy finds comfort in dwelling in the flashbacks which he seems to have no control over. Let us examine the first flashback in the play. Willy and Linda are discussing the boys' plans to move to Texas, and as a result Willy goes back in time to when the boys were young and the future looked promising. He finds security in the boys' eagerness to believe the words of their father, whom they adore. Note what Willy tells them:
Tell you a secret, boys. Don't breathe it to a soul. Someday I'll have my own business, and I'll never have to leave home anymore.
Note Willy's dreams are always put off or planned for some unspecified time in the future. His inaction is obvious, but crucially, retreating to the safety of flashbacks in the past allows him to briefly ignore the fact that he has never achieved what he set himself to achieve. Likewise, going back to the past reminds him of all the promise that the boys had and detracts from the failure that they have made of their lives.