Let's compare:
Definitely started the same. Identical streched fairing? Not an accident. But he made a significant and unique change at the payload attachment point.
S2 tank is originally a 6x14 vs the adapted 6x4. Detailing were left as he found it.
Stage separation was first via a separator, but he changed it to a docking port. He added RTG detailing right below it. Also the black tank below the port.
Retained stretched booster tank and landing leg mechanism but added RTG "grid fins" and detail to the transfer pipe.
Lower end is identical.
It definitely started out as the same vehicle. It is on a practical level sufficiently different and not a complete ripoff. He did however fail to credit the original author of the design that he modified.
Jryeem if you use a build from someone else, remember acknowledge them. Other than that, you did no wrong.