First, in playoff basketball, 2/3 of my frontcourt are elite defenders (LeBron's regular season stats are from his pacing). So Jokic is complemented much better than in Denver, where he is surrounded by mediocrity excluding Milsap. Horford matches up with the best offensive big, and, as you say, "a huge counter to the best centers"
Second, even if there is a defensive liability with Jokic, the offensive combination of the three will much more than offset any deficit. If a team goes small, I will have a counter from my bench.
Third, the "Jokic is a terrible defender" is a popular, misguided and exaggerated take, where people fail to take a balanced look at stats and blame Jokic for Denver's defensive woes. He is, in fact, an average defender.
Player/ Year/ DRPM/ DRPM Rank among Centers
Jokic
....2018/+1.95/ 21rst
....2017/ +2.29/19th
....2016/ +3.61/ 4rth
Last 3 years Defensive +/-: +3.1 +2.3 +2.1
2018 DRPM has him above the likes of DeAndre, Dwight, M Gasol, Drummond, Adams, WCStein, etc.
If he is badly hurting his team on the floor, there is NO WAY he puts up these stats DRPM exposes the KATs Okafors & Kanters of the world
People see his playing close to the floor and assume that he is ineffective. He is an average defender.
If you have KAT in your top 3, you must be completely ignoring KAT's obviously pitiful defense.
KAT
....2018/.044/ 63rd
....2017/-1.41/ 68th
....2016/-0.22/ 49th