It's not just "a landing job." It's "the default landing job" for a specific cohort. If an Ivy League graduate can't find a better job, this is one that they can always easily get just because they have a degree from a prestigious institution.

A default landing job means that you can always get this job if nothing else is available. It’s like saying, when you graduate from school you can always get this job to start

The word "default" is always concerned with the FAILURE to do something. To understand its most basic meaning, look up the financial meaning of the word (which I cannot explain to you here because iTalki would ban me if I used the needed words). So a "default job" is a job you automatically take if you FAIL to make a different choice. Likewise, on your computer, "default settings" are settings that you use if you FAIL to make any different choices.

The usual phrase is "to land a job". The phrase "landing job" sounds bizarre to me, but is grammatically correct I guess. "Default" implies "easy to get" and "landing" implies "easy to get" so for me "landing" is redundant. So I would have said "which is the default job" and left "landing" out. And I suspect at least one English learner would have been less confused with this.