From Reply #4, The system in the linked article was said to be the "same model, specs, brand".
When googling the Inspiron 3650 I didn't find any results which would suggest they had any variants which had a standard ATX motherboard or power supply. At least, not with the same model number.
It is, strictly speaking, possible to use an ATX power supply using an adapter like
this one, but, an ATX Supply does not appear to be able to install into the case.
I would have expected manufacturers to have stopped doing this but I guess not. To Dell's credit, at least it's more obvious that it's unique, unlike their famous proprietary supplies from the early 2000's where the connections all looked the same but they arbitrarily changed the pinout in a way that would cause using an ATX supply to fry both the motherboard and the PSU.