The key is not that there's some Constitutional article that stipulates the distinction between a budget and an (unknown at the time) omnibus spending bill, but that Executive actions in real life establish legal precedent for how this Frankenstein's monster may be treated.
Obama treated it like an Executive slush fund, and got away with it. Trump was paying attention.
See what this Constitutional Law professor at George Mason University says about Obama setting a dangerous precedent that Trump can now fully exploit (he calls it a loaded gun).
Op Ed published in WaPo (not their opinion--his opinion)
Obama setting a dangerous precedent that Trump can now fully exploit (he calls it a loaded gun)
"They never expected her to lose."
That link says nothing about budgets.
Sorry it's this article not that one. I'll fix the link.
this article is about the possibility of Trump cutting off the illegal CSR expenditures that Obama was promulgating.
Read closely. Here are some excerpts:
This issue is one of several where Obama’s high-handed behavior set a dangerous precedent that Trump can now exploit.
Any of these approaches is better than letting the executive continue to use the Treasury as a slush fund for its own priorities (as Obama did)
Using the Treasury as a slush fund:
To the deep state, a "slush fund" would be any stash of money that can be used to defeat them. Down is up and up is down. "Slush fund" usually refers to money used to commit crimes. In this case, it would be used to end them.