That's the end game, certainly, but having guaranteed income and additional programs doesn't hurt the experiment.
The idea is you can help pay for UBI by cutting the then-superfluous programs. But the benefit for people is in having the unconditional safety net, not in having access to only one program.
In the end, though, isn't that just an observation of how people behave with marginally additive wealth transfers? Shouldn't we have enough people who chose the annuity option of a lottery or had an unknown trust bequeathed to them to adequately study this phenomenon? I thought the real idea to be tested would be to take away the $1000/mo section 8 house, and instead give the recipients $1000/mo, and just see if they can more effectively use that resource.
Also, if we're making the usual assumption a UBI is intended to replace [most] needs assessment of state benefits, we need to test how people respond to having $2500/month value in various state cash and in-kind benefits replaced by $1000/month in cash (with the difference pocketed by people hitherto deemed less in need of support)
The idea is you can help pay for UBI by cutting the then-superfluous programs. But the benefit for people is in having the unconditional safety net, not in having access to only one program.