Sure, and there's no reason to not give Gretzky credit for his playoff performance. I'm talking specifically about the Conn Smythe award and the Stanley Cup, not what Gretzky himself did in the playoffs.
You can do a whole lot in the playoffs and still have zero chance for the Conn Smythe or the Stanley Cup, depending on how well your team performs.
In 1982 Gretzky scored 2.40 points per game in the playoffs, nearly the level he scored in 1985 (2.61). However, his teammates did not play as well, they allowed over 5 goals per game, and the Oilers lost in the preliminary round to a team they had beaten by 48 points in the regular season. Unsurprisingly, Gretzky did not win the Conn Smythe award that year.
Dude, you can't do ANYTHING in hockey WITHOUT your team. So when you, individually, set a playoff record for points (which aren't shared... but I assume you knew that) by scoring a staggering number of points on the way to your team winning the Cup, that's something you can claim as an individual just as much as any of Lemieux's Art Rosses that contributed to his Pearsons which contributed to his Harts. Setting individual records at the most important time of the year is great. Winning individual awards is great. Winning the Cup is also great. And that's why Gretzky's '84/85 was great, and largely demonstrably greater than any one season Lemieux has on the books.
Just, seriously, move on from your definition of "impressive" (or whatever your motivation for insisting we treat these guys in a vacuum to this extent) and please at least partially absorb how great a single season is if you accomplish
a list of things that no one (or in the case of the trophy collection, only Lafleur and Orr) has
ever done before - or since for that matter - resulting in single season resume/"historical record" that,
including winning the Conn Smythe and the Stanley Cup, could very well be the greatest season that any one player has ever had (hm to Orr, of course).
So what if Mario (or whomever else) never had an "equal opportunity"? Without an opportunity, you can't accomplish something, and thus you can't be allowed credit for having done it, and "fair" or not, it can't contribute to your "greatness" (or that of your season) by virtue of some woulda/coulda/shoulda.