Did we watch the same games 1, 2, 4, and 5?
Any time the Flames a lead or a modicum of momentum, Markstrom pulled out the rug from under the skaters.
Smith let in a goal from the opposing blueline and he still wasn’t the worst goaltender in that game.
Yeah, nah. I did a lot of yelling at my TV screen during that series, but it was directed at Markstrom basically only during Game 1 when he let in those two Hyman goals that no NHL goalie should be allowing, and then the Kane goal in Game 4 that, at the time, seemed like a dagger. Vast majority of the yelling was because the way the Flames were defending against McDavid and Draisaitl without use of their brains.
And it's not just about what I watched at the time. I've gone back and re-watched the highlights from every game just to make sure I'm not talking out of my ass because this topic gets me so hot.
Game 1 had some level of "Cam Talbot single-handedly losing Game 6 against Dallas" or "Brian Elliott single-handedly losing the series to the Ducks" kind of goaltending from Markstrom, but it really was restricted to only those two Hyman goals. The rest of them were the start of an even more important trend: Oilers getting inside the Flames defence with, or preceding, the puck. Two of the goals featured McDavid walking in on Markstrom, and one featured a Draisaitl breakaway.
Game 2 had no actual "bad goals." Arguably the worst was a low-circle one-timer by Keith while Markstrom was respecting near post against McDavid. But.... if McDavid is being allowed time and space right near the goal, you kind of do need to prioritize that if you're the goalie.
Flames allowed at least 5 guys to walk in alone on Markstrom during the game just based on the highlights. Teams should aim to allow zero breakaways in a game. That is obviously the number one issue that was developing at this point in the series.
Game 3: you could debate the first goal, but this game had Kane score a hat-trick off odd-man rushes. A memorable moment that has stuck in my memory since this game is the second of the three: Hanifin attempting to stand up McDavid at the red line and getting (as the kids say) cooked. Immediate two-on-one and a back-door tap-in. And this isn't even mentioning the big saves which Markstrom made in between on further breakdowns. Pretty clear at this point what the big issue is.
Thinking about the way you're describing the series, I would say you are probably basing your memory exclusively on Game 4. I think the Kane goal would have been disappointing to everyone, especially when Markstrom had already cost the team one goal from the drop with a puck-handling mistake. That said, this game featured the emergence of a new strategy for the Oilers based around their ability to get inside Flames defenders: stand a guy beside the back post, shoot it at his crest, and have him whack in the rebound. The Oilers went ahead and scored two goals this way in Game 4 and another in Game 5.
Long story short, the thing the Oilers "figured out" during the series was a) how to get odd-man rushes and breakaways against the high-press Flames defensive system and b) how to get inside the Flames' defence around the crease and then take advantage of that. Neither of these has anything to do with ways to beat the goalie. That's just how to beat a team in general.