Parallel LEDs in CoB-like array
There are three points that may explain how it is possible:


The led dies that are grouped are matched. Either they do the matching during the manufacturing of the whole led group, or they know they will always match because they take them from the same wafer and they made some tests that show it is enough to guarantee it works.
The substrate guarantees enough uniformity in the dies temperatures. So that if a led sees a little more current than another, the excess temperature is spread over the other leds and the current difference does not get much worse.
The Vf/If relation allows this. If you look at the Forward Current vs Forward Voltage graphs in the datasheet, you'll see that over the interesting range, the relation is actually almost linear (although steep). If you extrapolate these graphs to get the Vf/If of a three-die string, you'll see that if you have a Vf mismatch of +/-0.25V (over the whole string), it will lead to a mismatch of +/-50mA (relating to a current of typically 200mA for a single string).


So, if Cree measured that, for example, +/-0.25V was the maximum Vf mismatch they could have, considering different dies on a single wafer, and maximum temperature mismatch between dies once manufactured, they just have to account for the fact that, in the worst case, one string may see 150mA and another 250mA. That is manageable.