Firstly I missed *Purina* feed in your first post - thought we were talking 'senior feed' generically. And maybe your vet was talking about feeding it to your particular horse that you could 'shovel it in' & it would be safe. But it is still 17%NSC which, while lower than many feeds, is not all that low, and even on their site, Purina cautions feeding it to 'carbohydrate sensitive' horses.
Considering an acquaintance is currently treating her mare for laminitis after the mare got into the tack room of the trailer and ate about 6 lbs of a new bag of Purina Senior, yes, it can founder a horse... they weighed the bag to find out how much she ate and her vet said not to worry, it wasn't enough to cause a problem. She went out in the middle of the night to a mare in pain with bounding pulses in all four hooves. I am a fan of Purina Senior but it is certainly not something to 'shovel into them' without consequences.
I should have been more specific. She didn't mean that in a literal sense, and I should have been much more specific for OP's sake as well, lest there be a misunderstanding.
Dr. Hanna said that in response to my questioning her recommendation he get, through the coldest part of winter, a double portion of Purina to help keep his weight on him and to also put more on. I was concerned he'd founder on top of his weight issues. She wasn't meaning that in a literal sense, and I should have been much more specific for those who didn't know, because how could you know the context of a conversation someone else had with someone else. LOL That's my fault.