The ‘set’ message behaves differently from what you seem to expect. When in doubt, take a look at the Friendly Reference Manual.
The long answer is to send the output from jit.spill straight into the multislider’s left inlet. You can then safely lose the [prepend], [button], and message box. A [t b l] object would have been cleaner than the first two items, anyway.
use set if you want to setup the value of one slider with a two elements list (index, value).
use a long list (no "set" message prepended) if you want to setup the values of all the sliders in once, multislider considers the list has a list of value, the first element for the first slider, the second element for the second slider and so on.
and also, multislider is counting from 1, not 0.