Like I said before I think some couple of months already past lol
The reason for that is for one that Zack is completely healed up and the other reason is that I think the game is referencing the plate drop when Zack arrives. Mainly because we see the boy from the opening sequence sitting in Aerith's church and crying also just the way how the ending is cut plays into that idea, we are shown footage of Aerith looking up at the rain and saying she has suddenly a uneasy feeling just for it to cut to Zack who realizes Aerith left her church. From a narrative perspective the game seems to suggest that these things happen loosely around the same time.
Then what were Cloud and the others doing for 2 months after defeating Sephiroth and making it to the Midgar Wastelands during the ending of Part 1? How did the 3 months get erased to make Zack suddenly fit during the events of FFVII?
I feel like there's a distinct misunderstanding of FFVII-R's ending and when it took place, which has people working backwards to try and affix meaning to what's being shown in Intergrade/Intermission.
The fight against Sephiroth happened within the Singularity... Which then showed the audience that at the same time
in the past, the ending of Crisis Core was proceeding. Zack was making his last stand against the Shinra Army, which occurred in September of the year 007.
While Zack was fighting the final battle of Crisis Core, Cloud and the others were fighting against Sephiroth during December of 007, during FFVII-R.
The fight ends and Sephiroth retreats. The FFVII party lands on the outskirts of Midgar, in FFVII-R. Meanwhile, because the Whispers are gone, Zack emerged victorious against the Shinra Army and proceeds to make his way with Mako Poisoned Cloud to Midgar.
We see these two parties cross paths, going in opposite directions because they seemingly happen in the same location, at different points in time. One during Crisis Core. The other during FFVII-R.
Now, in Intermission, we see what happens after that ending with Cloud and the others. They are on the road, hoping to hitch a ride to Kalm.
We also see what happens to Zack. He goes to the church to see Aerith, to see her no longer there, people crying/mourning, and the flowers unkept and not looking so good.
These events cannot be happening at the exact same time because their previous events happen over 3 months apart. They are not in sync. The only way they could be happening concurrently, is if these are two timelines having their events play out in parallel.
It's a succinct and simple answer, that goes by the evidence presented. It is not complicated in the least.