No one is saying do nothing between applications. Just pointing out that it takes time to apply, opportunities coming around a few times a year and for many startups lack of funding can hurt/kill them.
The time it takes to apply for these seed programs is days not months. Many (including a16z START & 500startups) don't limit applications to "a few times a year" nor "accept you only at certain times of the year".
And the 'etc' of your claim "[t]here is really only YC, 500 Startups etc" hides a ton of options, including this a16z START program, programs from other VCs, TechStars, & a world of other seed-level options from other individual, syndicate (AngelList), or regionally-motivated investors. And, plenty have always-open rolling, or diverse staggered, investment windows.
So you never have to "throw away 3-6 months" or "wait 3-6 months" if you're making demonstrable progress on a good concept.
(Sure, once you've got a big burn rate, fundraising is likely to be a year-round time-suck with many-month closing cycles. But that's not the "even if you don’t yet have a fully formed idea and haven’t yet quit your day job" point this early-seed "market" targets.)
No one is saying do nothing between applications. Just pointing out that it takes time to apply, opportunities coming around a few times a year and for many startups lack of funding can hurt/kill them.