I believe assuming they are "just going alphabetically" and/or that there is some sort of implicit moderator managing the conversation qualify as non-obvious assumptions.
The vociferous objections on this sub-thread are rather amusing. It seems several people missed the bit where I said I was OK with the official solution at first, but kept being bothered about the problem construction. I doubt Andrew has any problem articulating the default 'correct' solution either. I was thinking about the problem construction anyway because I was annoyed that the problem as stated in Singapore includes several glaring errors of English grammar which were an annoying distraction, eg' I know that Bernhard does not know too' rather than '...does not know either.' English is an official language in Singapore so I was surprised that such obvious errors would be allowed to creep into a logic problem; it's akin to muddying up a mathematical equation with '-(-x)'.
That started me thinking outside the confines of the problem and about the implicit assumptions that were in the problem statement itself. The 'inability to piece everything together' is entirely imaginary on your part.
Given that this question is about humans, and discusses their behavior, it's clear you are making some assumptions about the structure of the question which are not present in the question.
That's certainly a reasonable thing to do.
What's not reasonable is to assume that the assumptions you make are going to match the assumptions of others. Likewise, when make assumptions there is a chance that the assumptions you make are incorrect.
That may or may not be the case here.
But the fact remains, you need to make some very strong assumptions for what you have said to be true.
You are correct, that the participants are people, with birthdays, and capable of speaking, is an assumption. You appear to be taking issue with that assumption. I suppose that is your prerogative, but it probably leaves you in the vanishing minority.
If you can't piece everything together then... well you're not suited to logical analysis.