I wrote a big paragraph but upon reading it back I confused even myself
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Smoothness generally refers to the actual plastic material the slider is made from. This can also be improved by lubricating. Below is an example, Gateron vs Cherry (Cherry have recently updated their molds apparently so only take this is an example, not gospel). Same design / shape, different smoothness:
It's the same with tactile switches. The Zealio slider has the same tactile bump shape as the Cherry MX Clear, but it produced by Gateron.
Smoothness is obviously highly desired, as the only 'resistance' you should be feeling is from the spring and the tactile bump, not from the roughness of the material.
With regards to buying Zealio - email him, he seems very quick with replies, or find him on Reddit or here and drop him a PM (ZealPC). I bought mine a few weeks ago at the $0.85 rate, and they shipped in a couple of weeks. Once that pre-order stage is over, the price goes up to about a dollar each which us retail. He should be able to advise what your current position is if you're ready to buy.
Hope that helps and made some kind of sense