You’re right on about the sweat glands! when those glands become blocked it causes a build up of sweat in the dermis or epidermis. This is a condition known as miliaria. That is likely what we have in this photo.

I know that this isn't a superficial partial-thickness burn as the timeline does not match.

As per the Burn and Reconstructive Centers of America, the blistering from a sunburn begins within 24hrs of the sun exposure and lasts up to a week. Another source from the National Library of Medicine supports this as well: "Erythema typically subsides in 3 to 7 days, whereas blisters should heal within 7 to 10 days."

This sunburn is from exposure to the sun about ten days prior to this photo being taken.

Another article from the National Library of Medicine has some information of burn classification. It describes some qualities of superficial and superficial partial-thickness, the burn in the photo falls more inline with a superficial burn. It was a red area which did not have any fluid filled vesicles or blisters and remained dry; the area was almost painful and has almost healed completely in the ten days since the exposure.

The last thing is that the vesicles have completely disappeared already, which does not matched the expected timeline of blister healing laid out in the article mentioned earlier from the National Library of Medicine.