The artist himself mentions earlier in the comments that they're based on dorudon.. I think he just tried to make them look as modern as possible, seeing as many cetacean reconstructions have them looking more like reptiles than mammals.
If I'm not mistaken, every non-archaocetid cetacean (that is, modern whales and dolphins and their most recent ancestors) lack(ed) hindflippers, while most archaocetids had them, if not fully developed hindlimbs. But some latter archaeocetids, those that gave rise to odontocetes and mysticetes, must have lacked them.
haha.. I do think they could've looked almost identical to a dolphin except for key differences such as the lack of melon organ, lower nostrils, conical teeth and hindflippers. I mean, most people try to reconstruct more 'ancient' things like dromeosaurs and oviraptorosaurs as being very bird-like, so why couldn't archaeocetes look like flat-headed dolphins?