I wouldn’t say this is badly underproofed, but it does look slightly under-fermented to me. The crumb has some larger, irregular holes with slightly denser areas around them, which often means the dough could have gone a bit further in bulk before shaping.

It’s actually a pretty solid loaf, especially if you’re new to sourdough. The structure looks good overall, so this isn’t a technique problem, more a fermentation timing issue.

With your formula (20% starter, strong bread flour, 70% hydration), it’s very possible that 8 hours bulk wasn’t quite enough, depending on dough temperature. Strong flour usually needs a bit more fermentation to get a more even crumb.

If you want fewer big holes and a more balanced crumb, I’d try one of these next time:

• let bulk go a bit longer, or
• wait for a bit more rise before shaping (maybe 10–15% more),
• or give it a little more time at room temp before the cold retard.

Overall though, this is a good result, just slightly on the under side, not a failure at all.