Well yes it does look like a size 18.5 to me, it certainly can't be a 16, and the owner may well be making the common mistake of measuring from centre of bb to top of top tube, which is how bikes were generally measured in earlier times. If it's 16" to the top of the top tube, then add another 2.5" to take it to the top of the seat tube and you have 18.5, which is what it looks like.
VooDoo started out in 1995, and initially all the frames were built in the USA by Altitude Cycles, which was Mountain Goat building frames for other brands in their factory in Chico, California. The Bizango was made from Tange Prestige Ultimate Superlight, the Wanga from Tange Prestige and the HooDoo from Tange MTB (i.e., a good double-butted 4130 cromoly, but not heat-treated like Prestige).
From 1997, the Erzulie was added to the range, double-butted cromoly built in Taiwan, and I believe (but haven't found confirmation yet) that from 1998 onwards all the frames were built in Taiwan, but still assembled into bikes in Chico until maybe 2000, using VooDoo's (expensive) custom options system. I labelled this a 97 or 98, but on further thought I'm not sure they made them in yellow until 99. Anyway, I think it's highly probable that it was made in Taiwan from generic double-butted 4130 of the same quality as used on a Cinder Cone or Lava Dome.
The geometry is similar to a Kona, but not identical. Some sizes had longer head tubes than same size Konas, and they had fractionally shorter top tubes, snaked seatstays and the cable routing works better. All of these factors make them slightly more like the way that design was going, with the slightly steeper top tube etc. Kona themselves froze their design as it was left to them by Joe Murray for 1994, and didn't make any significant change for many years. Whereas at VooDoo JM kept developing the design, so I would say at that time a VooDoo was Joe's more developed variation on his 1994 Kona design which Kona was still using.
