This is the contradiction within the rulebook. The first line says that ranged combat is possible provided that the elevated figure's own square is the only one to contain blocking terrain. By this rule, Barrier blocks LoF.

The second line says that LoF is not blocked by grounded blocking terrain. By this rule, Barrier does not block LoF.

If you do not see the inherent contradiction that accidentally found its way into the rulebook, then I'm not sure exactly what to tell you. However, the rulebook does contain a contradiction. The first line is correct, and the next FAQ will contain clarification. Until then, it is really up to each particular Judge how they choose to resolve the impossible scenario created by the rules, but I would advise ruling in favor of that first rulebook line, myself, so as not to make things more difficult for the players in the future.