In reality both ships the same size (gross tonnage and dimensions identical) - why one has a slightly higher deadweight I do not know, but that only effects the weight of the cargo it can carry, which was nowhere near max at 10500 tonnes on this occasion.
13558gt 147.75/156.93 x 22.8 x 14.0 (dr 10.275)m
DIJKSGRACHT 17381dwt, DANZIGERGRACHT 18143dwt
So, yes, they are technically wrong to claim she is the largest ship, but on the other hand no larger ship has been to Ipswich (unless they have missed a third candidate, of course)