Self learning 'big O' is a waste of time and effort. Never have I needed to know it to accomplish a task beyond interviews. Don't nest six for loops, reading lines from a file into memory each loop. Got it, learnt it, can do. Next problem.
This gatekeeping attitude is bullshit.
Understanding Big O notation is not that difficult and everybody knows it is a common question in interviews. Analyzing specific algorithms can of course be difficult.
Having a CS degree gives you a large advantage when solving problems like this, but that advantage is markedly lower when you are trying solve typical real-world problems in the private sector.
Self-taught or not, no one is able to derive an optimal solution to algorithm problems on the fly. You must have previously learnt the technique, and probably practiced applying it a lot.
Merging and comparing arrays as in that task also seems like something that might actually come up in a job.
However, I take the point of the other comment that the real issue here is doing it with certain speed requirements. That does make it harder, indeed, but still doable.