Grass tennis is a fascinating type of tennis that mostly relies on familiarity with the surface. There are very few players that are a natural fit on the surface and Djokovic certainly wasn't expected to become so dominant on it.
That's pretty similar to how people generally viewed Djokovic as he was never backed to become as good of a player as he ultimately did. Even so, there were some tidbits about his game that foreshadowed this type of success. He's always been a really good mover on the court and moving well on the grass is very important.
Even supreme talents like Alcaraz talked about how tricky it can be to properly move on grass and Djokovic always had it. Being able to hold under pressure is another thing. A lot of times you'll see yourself playing a tiebreak on grass and Djokovic currently holds the best winning percentage in tiebreaks out of all players replacing Roger Federer relatively recently.
Another thing where he could replace Federer would be as the winningest man at Wimbledon. He has 7 trophies so far but could win a record-tying 8th this year. He's certainly the top favourite for the event having more wins at the event than the rest of the Top 20 combined. In reality, only Murray ever beat him on a grass court out of all those that will play at Wimbledon this year.
As great as Murray played this year on grass, losing to de Minaur certainly proves that Djokovic could beat him if the draw pairs them up. If you're still not convinced then let's end on a fancy stat that we teased at the start of the story. Djokovic didn't lose a match on Wimbledon Centre Court for over 9 years now.
In fact July 7th will make it 10 years exactly so should Djokovic advance to the second week of Wimbledon, then he'll complete the unbeaten decade on the Centre Court. His last loss on Centre Court happened in 2013 when Andy Murray beat him in the final. It was July 7th, 2013.