Suppose that someone tried to naively derive the leap year by dividing the current year by 4. There are some years that are divisible by 4 but that are nonetheless not leap years. If you fail to account for this in your succinct implementation then you might introduce a leap day that doesn't exist at some point. I know the table implementation seems really dumb, and it's not that hard to look up a correct algorithm for this, but it's a good example. If you build a table from some correct source then you can simply use that table instead of running a calculation. There are fewer than 30 leap years in the next 100 years, by which time something cataclysmic will happen that will completely erase the need for your code anyway. At least, you will be dead before you need to revise the code again. So it's reasonable to build a function with a table given the time-limited nature of the code and the desire to be visibly and explicitly correct. Sure you could add a unit test with the table and test the calculation, but that actually adds more code to maintain than just jamming the table directly into the function.
I don't think they were stating that a simple table based solution was used, if it were, it would probably meet the bar for "succinct" because you could just write that as something like `if X in Y`. Rather, I got the impression that a lookup based approach was intended, but it was enterprise-ified (like https://github.com/Hello-World-EE/Java-Hello-World-Enterpris...).
Doing modulo 4, 100, and 400 isn't really that complicated to implement. I know there are even simpler ways to calculate this but come on, this is stupidly easy stuff. Then again we live in a dimension where there are Node packages for IsEven() and IsOdd() with tons of downloads.
Sure, in Node.JS, and with a specific calibur of programmer in a specific business context this is easy. Technical problems are easy to solve, but people and organizational problems are where the real challenges of engineering come. Sometimes you have to grit your teeth and implement something for the company you work for rather than for your own personal sense of correct.
