I agree that it's trainable, but to create a good icon a first timer can spend several hours. Several hours in which they could be doing work that they have trained for.
If one intents to be a sole developer then yes, they have to improve in all fields, but when one is in a team one has to let some things go, and let people better than them on that field do them.
Design isn't about drawing pictograms, so nice strawman. The type of design thinking implied in the comment is more about knowing –or at least have the faintest idea about what a visually cohesive system is and could look like. There's no such thing as a "sole developer" in software development because your code is inherently tied to function and that function tied to its design. The more you value that concept the more you "train" your understanding of modern graphic design.
I think:
1) Design is like a muscle, you can always develop it
2) Digital experiences [1] are all around us! And they play a more pivotal role in our lives as time goes on.
So, hit the gym (practice) & with time everyone can improve.
[1] As the https://mintdata.com founder here, I'm highly biased -- I'm not a designer but certainly aspire to improve along this dimension.