There’s a difference between daily fasting and a 7-day fast.
If you are fasting daily (so there is a period where you do eat something), you are getting nutrients from other sources and supplementing with the electrolyte drink.
If you are fasting for 7 days (not eating anything for 7 days) and drinking water only, you are diluting and depleting your electrolytes and would need to supplement for that.
I believe LMNT advertises as a daily supplement to food intake and doesn’t really promote extended fasts. So they’re going to recommend what’s generally safe and not get into a level that might be considered unsafe.
And no, you don’t have to chug water to consume 64 oz in the course of a day. That’s a pretty reasonable level of water intake and might be quite low.
I take 2-3 scoops of powder added to 64 oz of water, helps me get in all my water and supplements my electrolyte levels. I’m awake for about 16 hours a day… that’s 4oz of water an hour. Hardly requires chugging.
ETA: important difference! I use Ultima, which has a lot lower sodium so for me is a better choice for supplementing at that level since I get my sodium needs through food.
I think for extended fasts you do need, or at least would do better, with more than your one packet. I’ve done a salt shot with ~1100 mg sodium (I can’t sip it, the shot is better), with water chaser, and then ~640 mg potassium in maybe a cup of water over the next half hour with no ill effects. Also, a magnesium capsule around then as well. I’m following the advice I saw on some subreddit page, but I can’t remember which, of course! It was super helpful, but I only got some screenshots and can’t find it again. They suggest the 1100 mg sodium (1/2 tsp salt), 640 mg potassium (1/4 tsp no-salt) in 24 oz water with a 100mg magnesium citrate capsule 4x a day for an extended fast.