The MSM (and NVICs and policy letters) are guidance, not binding regulation. As is explained in the “disclaimer” of every policy document, alternative approaches will be considered.
The following is from a 2014 appeal of a mariner who was applying for QMED, the appeal was granted:
…this requirement in the MSM is only an example of service that would be acceptable, it cannot be interpreted as a statement of the only service that can be acceptable. Moreover, there is no regulation that would require you to provide service in an engine room comparable to that of a 100 GRT vessel or any regulation that an engineroom must have a “walk in” design to qualify for QMED-Oiler.
As noted, this was for QMED so it is not necessarily entirely applicable to DDE, but it is still relevant to your situation.
The requirement for DDE is for service as QMED or an equivalent position. You should provide enough information to support that your service was equivalent to QMED,