Short Answer - Yes, the rule only applies to skills which say "trained or no attempt"
Long Answer - Diplomacy does not require training, so the character doesn't trigger the bolded section's qualifier - "... where the skill restricts who can achieve certain results". Despite the fact that the character in question cannot roll a number high enough to pass the DC, there is nothing in the skill that prevents him from rolling.
Now, the best part as usual is the sentence after what you marked as bold. The GM can decide to limit aid others to only situations where the aiding PC is trained in the skill (a house rule I routinely use), but then again... even without the caveat from the quote you gave, it is always the GM's option to do that kind of thing.