Two people stated that the non-consummation of a marriage is grounds for a declaration of nullity—that is, an annulment. Surely this is incorrect, isn’t it?

You are quite correct, at least as you have stated the issue. A decree of nullity indicates that a marriage was never valid from the beginning. Since consummation is not required for a marriage to be valid, non-consummation is not itself a ground for judging a marriage invalid from the beginning. That said, there are two caveats that need to be made.

First, if the marriage remains unconsummated for an unusual period of time, it may be due to something that did render the marriage invalid from the beginning (such as antecedent and perpetual impotence, (Code of Canon Law 1084 §1). In this case, the non-consummation is not the grounds for nullity, it is an indicator that there may be some other condition that might be grounds.

Second, while non-consummation does not mean a null marriage, it does mean it is dissoluble. It is consummation that achieves the “one flesh” union (1 Cor 6:16) which Jesus says makes marriage indissoluble (Mk 10:7-9). Consequently, a decree of dissolution can be granted for a non-consummated marriage.