This appears the documented method to use the "other" bootloader.
There are two bootloaders:
-
the "main" one that is invoked by pressing reset (and keeping it pressed until leds start flashing) immediately** after** (I guess that your 0.5s counts for immediately) having inserted power
-
the "backup" one that is invoked by pressing reset (and keeping it pressed until leds start flashing) before inserting power
99% of the time the second method is used as it is much easier to perform, the immediately in the first one is difficult to replicate in practice.
It is very possible that in your case the "backup" bootloader wasn't working properly whilst the "main" one did the trick.