Velox/Jantex tape is some sort of textile strip dipped in Tubasti rim cement so both sides are adhesive.
Tubasti rim cement isn't the greatest of cements to begin with and if you'd combine that with layers of a different brand of cement then it would even become worse than it is already.
Who ever wrote the copy for this add either never saw Tufo tape from up close or Jantex or both. Either way the similarity ends with the word "tape" :http://www.bikeman.com/bicycle-product-reviews/new-product-introductions/2396-velox-jantex-belgian-style-tubular-gluing-tape
Not sure why the tape is used, the only plausible reason I can come up with is that it essentially adds another "substrate for the bottom (rim side) and top (base tape side) to "cling to. It probably also makes it easier to remove the tubular afterwards as the cement is likely to separate at the tape instead of at the rim or base tape.
One could argue that a stronger cement could be used but that would very likely compromise the bond between the base tape and the tyre proper.
Description of gluing method:
BTW. only scrape the base tape if you know for a fact that it has been coated. Not sure what's best for CX but for road I'd recommend not touching it even if it does come with a latex coat. Currently that would be the likes of FMB, maybe some Dugast and older Veloflex Record and SC.
Others like Vittoria use a different kind of coating which contains fungicides and what looks and feels like a thin coat of Aquasure or similar. Leave it, it won't do any harm quite the opposite.
That being said, I do not know of any Belgian CX rider that actually uses this method but if there should be one on this forum. I'd sure like to know more as to how all this came about.
Until the jury is out on this one, we still have lots of truly Belgian chocolates for you all. Not to mention all sorts of wicked brews too......