This assumption that there is some "finite budget" of man-hours and money for a company choosing to retool code for a universal binary version of software that somehow wastes resources to be used elsewhere seems very weak to me.
There is an existing tool (Xcode 12 and later) that will do the heavy lifting up front, so it's not like developers are starting from scratch. From what I understand reading over Apple's developer page, a universal binary is built automatically by Xcode when making a clean build and choosing arm64 from Apple's standard list of build architectures. It gets more complicated as programs deviate with custom makefiles and build scripts and probably a bunch of other stuff I sure don't know about, but that's why it takes far more complex programs longer to be ported. I wouldn't be surprised to learn that Sidefx has been working on a universal binary since day one.
Adobe gave us Illustrator, Photoshop, Media Encoder and Premiere in universal binary rather quickly. We're still waiting for After Effects (it's been in beta as universal binary since Oct 2021) but I assume AE is their most complex program to retool. Granted, Houdini is even more complex than that.
Universal binary ports on programs like Houdini and Cinema 4D and Blender, I'm sure, are not easy. But even if Apple users represent around 10% of their entire user base, any company will happily take a 10% increase in revenue
if the resource investment is some smaller percentage of that. Especially given that people who buy Apple products tend to spend more money. As an example on the iOS side, Apple's app store generates nearly double the revenue of Google Play while the worldwide user base of iPhone users is dwarfed by phones running Android. Ignore Apple users at your own financial risk

They're vocal, they're loyal and they evangelize.
Anyhow back to your assumption (or perhaps you are just being devil's advocate) that a company is somehow "robbing Peter to pay Paul" doesn't have much merit, in my opinion. If a company felt it was a waste of time and resources that would not pay off, they wouldn't bother. Of course I could be wrong, but personally, I'm confident Sidefx appreciates that a universal binary of Houdini will be worth it and that they have been working on it for some time.