There are two topics at play here being conflated, mostly by the OP:

1) Should DL have held the flight or not let it leave five minutes early? I would say probably not, although pretty clearly, operations (and not the station manager, despite what OP suggests) felt otherwise. I have found the jitney folks to be relatively incompetent/uncaring myself, although that's also .0001% of DL's workforce. It seems like the issue, more than anything, was the delay in DUB. OP repeatedly avoids mentioning what the cause of the delay is in DUB, which is likely telling.

2) Is this proof DL are a bunch of Grinches/don't care about passengers? Absolutely not, no matter how frequently the OP doubles down or insists that other people are wrong. Would it be any different if this happened on July 10 as opposed to December 10? Would they be ruining summers? There's no good time for a delayed flight. This happens every day on every airline. If the delay in DUB is really DL's fault, collect your 600 EUR and move on.