Poetry can be challenging to read, especially because poems often have multiple meanings. I'll put forward my interpretation of that line, and I welcome others to share their thoughts as well.
You're absolutely correct that the author is basically saying:
We never heard where they went or whether they were still alive.
The meaning of the word "mock", however, I think is somewhat complicated. I see a couple of interpretations, and I think the author may have meant a little of each:
The word "mock" very literally means "treat with contempt or ridicule". The soldiers who are still alive have found the experience of war to be so awful that they now talk about their contempt for the support they received from the women.
The word "mock" still means "treat with contempt or ridicule", but less literally. The soldiers are not actually talking amongst themselves. Rather, war is so awful that the mere fact that they must serve as soldiers in a terrible war shows contempt for the love and affection they were shown when leaving the town.
The word "mock" means "disappoint or frustrate (the hopes of)". In this interpretation, the fact that the soldiers must fight in a war frustrates the hopes of the women who wished only the best for the soldiers leaving the town.
I think the author may have meant a little of each of these interpretations. I don't believe he meant that the soldiers were laughing and joking about the flowers, however, since that doesn't match the somber mood of the rest of the poem.