Raising a Series D is an attempt to permanently increase the velocity of their enterprise. The change in the cap table is negligible if they succeed.
Raising debt right now would be a massive unforced error given they were able to shore up half a billion dollars in funding. If the funding well were dry then it'd be a different story, but that's not the case.
Can you imagine if they raised $445M of debt?
When you sell shovels, even slight downturns are massively exaggerated because the shovels can be resold, and if some of their customers go under, it's not just that demand that goes away, it's all the demand that can be satisfied by their customers purchasing second hand servers from the bankruptcies instead.
No-one can really say how long this bubble will last. Might pop tomorrow, might be a decade. But if Oxide funds inventory on debt, and the bubble pops, they go with it. Whereas if they fund on equity, and it pops, they will come out the other side just fine. The cost of this is that the old stockholders have a slightly lower upside.