Show All Tabs on iOS definitely does not reload tabs. I know on macOS it used to force all tabs to load but I had the impression lately that it wasn’t doing that anymore (though I rarely use this feature).
iOS reloads them ( or partially load them ) as you scroll through it to get a screenshot. I tried it just now. Or they somehow saved a Screenshot of it. ( But then there is no reason why macOS safari isn't doing the same. )
iOS saves screenshots of tabs. I scrolled through my own and found a bunch of old tabs (didn't realize I had so many) that just showed pure black.
macOS saves screenshots too, to show when hovering over a tab. Curiously, it doesn't seem to use these saved screenshots in Show All Tabs. Also I just tested, Show All Tabs on macOS is no longer loading unloaded tabs. I have a window with 13 tabs right now, only two of which I've visited since launching Safari, and the others just remain as blank in Show All Tabs.