It could be for a number of reasons.

1. OP wanted to emphasize what it is like to interview at a company which has a lot of resources to invest into interviews - one of the things mentioned was committing multiple people for a full day to just interview him, as well as willing to bear a high initial cost (travel, hotel) to interview him.

2. OP wanted to detail the interview process of such a financially successful company: who they hire, why they hire that person, how they find and select candidates that have contributed to that high financial success.

3. OP wanted a crafty title beyond "My interview at Apple"

4. More likely to be clicked on, shared, commented or voted on (i.e clickbait).

5. They have $700B, and even they can't they afford more than a "thanks but no thanks."

"A few people asked me to write about my experience of interviewing for a job at Apple—the 700 billion worth company—so here it is."

I still don't understand, but ok, thanks. That's a great descriptor of Apple (of which there are many), but doesn't point anywhere toward a rationale for titling an interview experience as "700 billion". I'm not trying to say titles need to make logical sense, but they usually should make some sense

it refers to Apple's current Market Cap

I think he understands that, but is wondering why that is relevant for the topic at all.

it's called link bait.