Back when the season began, the blueprint that Doc Rivers laid out for his team to contend for a championship was to be top-10 in both offense and defense.
That goal is still within reach, as despite the fact that the Sixers have struggled for most of 2023 (19th in defense since Jan 1st), they still have a decent chance of finishing the season with a top-10 ranking. They are currently tied with both Minnesota and Phoenix at 113.6 points allowed per 100 possessions for the 7th/8th/9th spots, and just barely ahead of New Orleans at 10th (113.8), but there's then a decent drop-off before you get to 11th (OKC at 114.1). With only 12 games remaining, and with the Sixers looking like they'll have something to play for right down to the wire, that's a lot of ground to lose in a short span of time.
There's still some question as to exactly how much more effective the Sixers' defense will be in the playoffs, when they're fully dialed in, and specifically when Joel Embiid is locked in on every possession. Embiid, when fully engaged, has an ability to clean up his teammate's defensive mistakes and shortcomings in a way that almost nobody else in the world can, which should make them look better than they have been at various points this season.
I certainly think there's a chance they can be "above average" defensively in the playoffs, perhaps even "good", but there are enough players who can be exploited, and enough mistakes that are made, that even when the Sixers raise their intensity up to a playoff level I'm not quite sure I see a dominant defensive squad. And more importantly, I think there will be specific matchups that you worry about on that end.
Over the years, you can certainly make the case that defense more directly correlates with the eventual champion than offense does. In the last 20 years, only one NBA champion (the 2017-18 Warriors) ranked outside of the top-10 in defense, and they were 11th, and had ranked 2nd in the league the previous year. If you average out all the offensive and defensive rankings of the last 20 NBA champions, teams on average rank higher on defense (4.75) than offense (7.1), and 14 of the last 20 NBA champions had a defense ranked in the top-5.
So, if the Sixers' defense is a step below your typical NBA champion, can the offense be dominant enough to pick up the slack?