Endurance doesn't always equal physical durability. When she says Endurance boost, she can just easily mean it surpassed her ability to boost her endurance against decay. "Cathleen Bate won't decay." She wanted to endure decay and failed to do so.
Endurance can mean durability yes, but I don't see anything to suggest that means she boosted her physical durability in this case. Is there any reason to assume physical durability is the correct answer here? Considering the non physical nature of the Quirk, it's better to assume she endurance against decay, than endurance/physical durability against impact.
However a fan translation had her note that it was physical enhancement, I'd like to see the raw japanese translated to get an idea on what she truly said.
There's also the possibility he slowed his decay down so as not to kill her before he can take the Quirk. From what we've see here, AFO isn't a instant steal. While Shigaraki's can rip through people
like this. And considering her decay statement came after he took his hand off her. I think it's clear that he is also holding back.
Since he could've done this, but he wouldn't have been able to take her Quirk. Which could imply her rule did absolutely nothing, and stopping decay in general is something that surpasses her power. Though this doesn't really mean anything in regards to durability or not, just an explanation on why it was slower.
Long story short... I don't know the answer.