The phrase on the side is a tricky one, because it has several possible meanings.
If I had a cyst on my torso, and was going to have it removed next week, I could tell you that I was going to have surgery on the side.
Of course, in this context, we are referring to one of these two idiomatic meanings (definitions from NOAD):
on the side
1 in addition to one's regular job or as a subsidiary source of income : no one lived in the property, but the caretaker made a little on the side by renting rooms out.
2 secretly, esp. with regard to a relationship in addition to one's legal or regular partner : Brian had a mistress on the side.
There's nothing furtive going on here, so that rules out meaning #2. What the writer really means here:
Maybe I should take up surgery as part-time work.
In this context, the writer is trying to say that the two could learn a lot from each other: what comes natural to one is not nearly so simple for the other.
One more important detail: no one takes up surgery on the side. We might sell real estate on the side, we might tutor or teach on the side, or we might work in a restaurant or retail store on the side. But surgery is a skill and profession that requires years of intense training. So the writer here is being humorous. He doesn't really intend to take up surgery as an extra job, but he's reinforcing the notion that sometimes it's easy to forget how difficult something can be after we have been fully trained.
I've always known this expression to have both a literal and figurative meaning, and have seen it used in both scenarios. For example:
Lit: "We covered a lot of ground on our hike today." - meaning we physically travelled a large distance over the course of the day.
Fig: "We covered a lot of ground in Science class this semester." - meaning we dealt with a lot of information in Science class.
So I would say that yes, the usage of the expression as an idiom with respect to a topic is valid.
Best Answer
The phrase 'all in' originates in poker and other gambling, where it means "I am betting all the money I have".
This has become a metaphor recently to mean "I'm totally committed to something", or "something is the most important thing to me".
When using a verb like your example, you can use either the infinitive (to hear) or the gerund (hearing), often with prepositions like 'on' or 'for'.
means
All in or all-in (MW, definition 2)