Not an issue, you need to read the descriptions more carefully ;-)

Well, take isMerge('codewars', 'cwdr', 'oeas'). If you try to arrange the three strings as in the description, you'll see that it doesn't work for 'cwdr':

s      = c o d e w a r s
part1  = c   d   w    r    <-- becomes "cdwr" instead of "cwdr"
part2  =   o   e   a   s

Order is important. And sorting everything discards order.