Frank Miller is a prolific writer and avid Batman fan, but his stance of how this Bruce would be at that age is not in line with "Canon" Batman. By that, he does have the intense, solitary nature, disdain for crime and sparkling anti-social personality... but they are turned up to 11, added with some old-man bitterness/disillusionment, and grounded in the grim/dark "gritty" so-called "Realism" that the mid-to-late 80s, Post-Rambo world had come to expect from its "heroes". This, essentially, was a bit of a counter to the 1960s "campy" Adam West Batman, and the early 80s "white bread" cartoon version of super heroes [including the "Super Friends" Batman].
Now, originally [as in his 1939 debut] Batman was darker in tone and a bit more liberal with killing, but that was due to his connection to the pulp-heroes at the time. This didn't even last 3 years; the no-killing was instituted fairly early on, most firearms were removed, and by the 40s, while a vigilante, his reasoning was for the protection of human life. These elements became fixtures [especially post the game changing Seduction of the Innocent ] as the funny-book 40s, the Norman Rockwell-eque 50s, and the campy 60s went on. Batman's tone got slightly darker in the anti-establishment 70s, but by the optimistic 80s, he was still a firmly "Good guy" hero. Due to the impact of various social conditions in America, the whole "grim dark" aesthetic and muscular, attitude carrying, brutal heroes became the norm.... and Batman (with his already darker origins) was a natural fit for these changes. This is one reason why Miller's TDKR resonated as it did, especially as mainstream comics were becoming more mature in their stories at that time.
Due to the impact of Miller's work and the aforementioned social changes, his version of Batman--- a brooding, sullen, something of an anti-hero, grim Dark Knight--- became somewhat of a "Default," and began influencing almost every version of the character that came after it. Even the 1989 Tim Burton version of the hero, which was arguably more balanced portrayal in some ways, took some inspiration from Miller's dark tone and grittiness; note the guns in the Batmobile, and Bruce's tendency to silent violence in the first film. To mention nothing of the number of people the Joker killed in that movie...and this was [mostly] a kid's film in the 80s!
So while not "canon" in itself, Miller's version has, to a degree, help shape the future canon of Batman. Most versions of Bruce still hold darkness and intensity, but not to the grim-dark, anti-humanity level that Miller's pessimistic, nearly hopeless and pitiless Batman takes it. The DCAU version probably struck the best overall balance in this regard:
That Batman---voiced for 20+ years by Kevin Conroy, and which many look to as the definitive version--- is intense, controlling, has issues and can be brutal... but he is no killer, has obvious compassion, despises guns [even with rubber bullets], and does what he does not to punish criminals alone, but to prevent the tragedy that happened to him from happening to anyone else, where he is able to. Oddly enough, Amanda Waller, a hard-nosed often antagonist and well-intentioned extremist in the DCAU probably said it best in the Justice League Unlimited episode Epilogue :
I've known Bruce Wayne for over fifty years, and I've been keeping an eye on you your whole life. You're not Bruce's clone, you're his son. There are similarities, mind you, but more than a few differences too. You don't quite have his magnificent brain, for instance; you do have his heart though, and for all that fierce exterior I've never met anyone who cared as deeply about his fellow man as Bruce Wayne, except maybe you...
So no, Miller's version Was not canon before, and in many ways still isn't totally canon, but it does fall "in line" with current canon to some extent.