How insulated is guaranteed funding from an economic slowdown?
Large universities that are offering guaranteed funds often don't guarantee where those funds will come from, simply because they cannot: the money doesn't exist yet. In my graduate program, this is what guaranteed funding meant:


Your first year was covered on a T32 training grant from NIH. This allowed you to rotate in different laboratories without those labs needing funding for you. The NIH provides these grants specifically for this purpose. If all the training grant slots were filled, some students would be admitted directly to a lab that was able to fund them starting their first year.
After rotating, you would join a lab. This lab would basically need to have funding for you, or else you would have to teach. It is important for students and their PIs to talk about funding, since some labs have new grants or several grants and can pretty much slot out a PhD student's entire time in the lab; others have funding expiring soon and the student and advisor need to have plans if that funding falls through.
At any point, most students would apply for outside funding, and some would get it. Even though funding was guaranteed, applying for and receiving grants looks good for your resume.
Students were required to TA at least one semester; many would TA more if they were interested in teaching or to cover breaks in funding in their labs. Usually these would be negotiated with the students and students could time their TAship to coincide with a low funding period in their lab if necessary.
In rare cases, students in labs that failed to secure funds continued their research with funds from another lab, perhaps changing their research project. The program would consider these hardships when granting the PhD degree, and it might result in a thesis that's less cohesive - that's just part of life.


Note that most of these steps didn't really involve the guarantee, it just involved there already being money around the labs that make up the program to fund students. Guaranteed meant that the program would be your advocate and would find you funding of some sort if other sorts fell through. They would find a way to get you a TAship if you couldn't get an RA. They would ask someone else to give up their TAship for you if they could get funding from their lab. They would find a way to stretch funds from one part of the budget to another to make sure you graduate. They would use training grant money for you to finish your final semester. The money wasn't in a pot set aside for you at the outset, but in the fungible world of money, the university was committing to support you. Guaranteed did not mean you could work in any lab of your choice and work towards any research project of your choosing, but it meant that lack of funding would not be an absolute reason for you to be unable to continue in the program over your first 5 years.

Every program will differ in terms of what types of support are available: TAships, RAships, PAships, fellowships and other funds. Funding guarantees are important, but so are the sources of those funds, and this should be an open conversation throughout the application process.

As raised in the comments, there are situations where even 'guaranteed' funding can be revoked, since the guarantee almost certainly comes with basic strings attached such as continued progress in the program, not violating a code of conduct, etc. That said, this sort of guarantee from a reputable university program is putting their reputation on the line to get you funded. They will only break this promise if necessary or justified. No employer is likely to ever provide you a stronger guarantee.

I started graduate school around the last recession; ultimately, the recession and associated funding cuts meant in some of the years after me the program admitted fewer students than typical over a couple years, but no one went unsupported.