How was Robin able to stand despite being cuffed with Seastone?
Read that chapter (169) more carefully then...



Here, Luffy stands in Crocodile's Kairoseki cage.



Then he states that he has no power. Well, of course, since the cage is made of Seastone.



Then Smoker kicks his ass and nails him down. And I think regardless of Seastone any of us would have problems standing if Smoker pushed his weapon to our chest.

So to answer the question: Robin was in a weak state because of the Seastone handcuffs back then, but she could stand because nobody pushed her down.