I did hear back from Berroco regarding the term "welted seam". Another knitter who had the same question posted Berroco's reply in a comment to my original entry. (I imagine they get this question ALOT!)
Basically, it refers not to a type of seaming but to the look of the seam. You sew the pieces together using a method that results in the seam showing on the public side of the work. Any method...
You could do the mattress stitch with the wrong sides facing; or do an overcast or whip stitch while holding the wrong sides together.
Yours in knitting,