Restaurant staff eating leftovers

Ordered buffet and take away

they can be happy that I wasn't there

In what way? Since you obviously do not know any legally relevant basis for this case yourself, you could not have done more than looked puzzled.

Before tackling this question from a legal point of view, you first have to clarify whether the statement about the cold chain is correct. You yourself seem to be unsure about this and I myself cannot answer this question with expertise.
In fact, I think it is possible that takeaway food can be dangerous. Then one would have to add the question to what extent the buffet service would still be responsible for this or whether it could not be completely irrelevant.
But I also think it is possible that the cold chain was only a pseudo-argument and that people simply wanted to forbid taking them with them.
In between, there is also the possibility that the buffet service was wrong and wrongly assumed to be dangerous.

But even if you assume that the cold chain is wrong, you and your wife may not care. You are not a contractual partner, so you have no claim against the buffet service. The wedding couple alone could make claims.

The possible claims of the wedding couple would be limited by the damage actually incurred. This should at most consist of the purchase value of the meals otherwise taken away. It can't be about too much money, so I wouldn't want to start a lawsuit. At least not after payment has already been made. Otherwise, one could think about holding back a small portion and let it depend.

Anyway, the question still arises as to whether there was any entitlement to take away or whether it might even have been excluded. Especially in the case of a buffet, where there is no upper limit for the food provided and the buffet is refilled according to the actual need, it should speak against the traffic custom if the wedding party spontaneously uses this flat rate and delivers outside the home.

Or maybe the service staff was just hungry themselves.
You can't know any of that.
But I wouldn't get upset about that.