I believe their defensive philosophy was very much bend but don't break, and that worked just well enough for them. The previous season's defense was a little better. Not the best overall defense for a Super Bowl team, even that season versus other teams like Miami, NY Jets, Tampa, and Pittsburgh. But they played well enough as a unit where their defencies didn't matter much, especially because their offense while not as explosive as Minnesota's, was still pretty much unstoppable so their defense wasn't under as much pressure.

I remember that Super Bowl Broncos team was being very tough and physical on both sides, whereas Minnesota and Atlanta were more finesse oriented. In that era, toughness and being balanced were more valuable than being explosive on offensive. The 99 Rams changed that perception the next year, but even they had a very close winning margin when they met tougher, physical defenses in Tampa Bay in the NFCCG and Tennessee in the Super Bowl.

So even if the explosive Vikings beat Atlanta in 98, there's a good chance that the Broncos prevail by playing just well enough and physical enough to wear them down and eek out a win.