I think they saved the budget for season 3 because according to vivienne it’s the best animation she’s seen, which means it’s gonna be like invincible where the animation looks really great but scenes where it looks janky and weird.
I'm not sure what the agreement is for Helluva Boss, but I get the feeling the budget for the shorts is not associated with the Amazon deal.
Looking through the credits, I saw no mention of Amazon for the Barbie short. Which means they did not fund it. Even if all you provide is money, you get a name in the credits. And the fact it is aired on YouTube and not Amazon, who would have exclusive distribution rights if they were involved with it even on a fiscal level, confirms that they are not responsible and thus the budget is not shared between shorts and the main series.
The shorts are made entirely through Spindlehorse production pipeline which means it is Spindlehorse alone funding the shorts. That is SpindleHorse's budget.
Meanwhile Amazon ordering more seasons makes them a financier of the main series, at least. In Hazbin they have more control based on the fact they could tell Medrano to change anything about her series.
Additionally, Hazbin was animated in Amazon's studio effectively making Amazon the Legal Author of the series while Medrano gets to splish-splash in their pool with their finances, their animation pipeline, and their tools. This could be shared between the Spindlehorse Studio, but Amazon has partial ownership of the product that is the animated series, though they would not inherently own the IP.
Meaning Medrano could go elsewhere later to retell this story, but would be required to make changes in visuals and possibly even format to not overstep Amazon's ownership of the main show. Which is why her mentioning a webcomic series is important. She wouldn't be able to reanimate the series without encroaching on Amazon/A24's copyrights of the show.
Amazon's Helluva Boss hasn't been released yet so I can't look through the credits to determine how much of it is run by Amazon and how much is still in Medrano's control. So I assume they are mainly an executive producer with financial backing and distribution rights.
So the Shorts are reflective of Spindlehorse/Spindleroo alone. Their finances, story, animation, etc are not tied to the Amazon deal. What you see on screen is the best Medrano can afford without Amazon cleaning up after her. And based on that short, I see why she had to run to Amazon to keep Helluva Boss going.