Walmart doesn't pay a wage that would allow its employees to be housed, showered and fed as they report to work and yet, they get employees that are and all the benefits that come with it. They should be hiring exclusively from the homeless at the rate they wish to pay.
As other comments have noted there are income limits for SNAP. A single person working at Walmart full time would make too much for SNAP. So would a family of two with both people working at Walmart. It looks like same for a family of two adults working at Walmart that has 2 kids (who do not work). A family of two adults with no kids with only one adult working would be right near the boundary.
Without more information on the distribution of family size and type among Walmart workers the Sanders numbers aren't very useful. To illustrate how much family composition can matter, I've known Mormon families that would have qualified for SNAP if the working parent had taken an entry level engineering job at Google, and no one would ever seriously say that an entry level Google engineering job does not pay a living wage.
Why not underpay people if the taxpayers will pick up the slack?
Why not dump waste in the rivers if the taxpayers will clean it up?
It's not like there is a penalty for doing so, beyond the occasional slap in the wrist.