Of course you can still check signatures if you wish. Heck, sandstorm even provides instructions on how to do it on their installation manual: https://docs.sandstorm.io/en/latest/install/#option-3-pgp-ve...
I'd also argue that secure defaults are most important for folks who don't care. And there are more secure options: the package managers.
So sandstorms installation guide should in your opinion just say "wait a couple of years and maybe someone will have packaged it for your distro"? Yes, that sounds excellent way of getting users. And hey, the most secure way of installing software is not to!