I think he's referring to how when you're really dehydrated, you give people IV saline because it is a much faster uptake. I honestly don't know the mechanism and am not looking up why right this minute, but my inference from his comment is he's claiming the COVID vaccine hits way harder than a normal flu vaccine.

I've already had an anecdotal report from someone who received the vaccine that it 'floored her' for an entire day.

I hope this helps your understanding.