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> Staying in a seated position for too long allows gravity to pull more blood down into the pelvis

I could concede that more blood might end up in the pelvis, but surely this is not the reason?

Why "surely"?

Because the torso is upright either way, and gravity doesn't care if you're sitting or standing

Gravity does the pulling, and sure the blocking is caused by the lack of movement and the sitting position that obstructs the free flow of blood.

I guess we could say it's not the gravity itself, and it would be correct, but also like saying it's not the bullet that kills you, it's the hole.

Gravity pulls blood down, your circulatory system works to more evenly distribute the blood but is impaired when sitting.

The same phenomenon is observed when sitting on an airplane for a long time resulting in slightly enlarged feet due to blood distribution.

So then wouldn't the mechanism be that less blood is able to flow upward due to pressure on the skin?

Blood flows in veins due to contractions of surrounding muscle. When you're sitting, your lower body is largely relaxed, which causes blood to pool in your lower body. It's a serious (acute) issue in people who are bedridden for long periods of time. I suppose the article is suggesting there's a downside to chronic, low grade exposure as well.

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