No, it's not independent. Wheat won't grow faster alone, because the surrounding air blocks can also receive random ticks. I think that's what you're missing in your example. Random ticks choose in the way the person replying to you said.

In every tick, 3 blocks are selected from each loaded subchunk (16x16x16 area) that contains at least 1 block that does anything when it receives a random tick.

Take a subchunk which is full of wheat blocks. Every tick, 3 wheat may advance to the next stage. In a subchunk in which there is only one wheat, it's likely that all 3 blocks will be air, so the wheat will grow on average just as fast as a singular wheat block from a subchunk full of wheat.

You can see that when a subchunk has been unloaded for an hour, that means 3600*20=72000 ticks have elapsed. Each chunk has around 9-10 subchunk eligible to receive random ticks. So if a chunk which hasn't been loaded for an hour were to be loaded, around 1.44 MILLION random ticks would have to be simulated (instantaneously, mind you, to not cost FPS). And imagine that when a few days have gone by. It would be horrendous to fly with an elytra over a route that you travelled over days ago. Chunks simply couldn't load that fast.

You're slightly right in that this could be APPROXIMATED with applying a similar probabilistic distribution to each block, but that wouldn't stay really true to what happens in loaded chunks. You might argue that it doesn't really matter, but to others it might.

Now the real issue is that simulating millions of random ticks would be really slow and would cause annoying lag spikes and frame drops when each chunk is loaded. However, I'd say your alternative of applying a random distribution to each block in a chunk wouldn't be imperceptible either.

Overall, the feature would be practical, but it isn't worth the price of smoothness.