I think they are telling you to smack it so early because they want you to see the pack is inflated, and therefore viable BEFORE you even start brewing. That way if you have a dud yeast, you can abort the brew until you get a replacement yeast. Once you brew, you need to get live yeast in the wort before something not-yeast starts growing in it.
Wyeast also prints a manufacture date on the pack. I’d suggest give it an hour to inflate for each week that’s past since manufacture. If you really want to walk on the wild side; you don’t actually have to smack them at all. That pack just has a little snack for the yeast to nom on; they eat the snack and fart the pack up; it’s just a gimmick that proves the yeast is alive.
The NB instructions are the fail-safe path to drinkable beer. Once you get a bead on their instructions, and start to learn more advanced techniques, you’ll find yourself using the instructions just for the kit inventory and hop schedule. For example, many regulars on the forum will say you need a “yeast starter” when using liquid yeasts for most of the 5-gal recipe kits. NB doesn’t even mention starters in their instructions. But go ahead and brew that that first batch without; you’ll be fine. Learn by brewing.