I don't know that I necessarily agree with this side, either
The reality with the Penguins right now is pretty simple: they're an old and mediocre roster with a coach years past his expiration date. How they got to this point is more complicated. The situation Dubas inherited was not just as simple as "he just needs to surround the core with good players".
When Dubas took over the Penguins, this was their fully healthy roster to end that season:
Guentzel-Crosby-Rust
Zucker-Malkin-Rakell
Heinen-Granlund-Carter
Poehling-Bonino-Archibald
O'Connor
Dumoulin-Letang
Pettersson-Petry
Joseph-Rutta
Kulikov-Ruhwedel
Jarry-DeSmith
Their bottom-6 was a downright mess and the defense's puck moving wasn't nearly good enough to make Sullivan's system work. Zucker, Jarry and Dumoulin were pending UFAs while Petry ($6.25 million), Rutta ($2.75 million), Carter ($3.125 million) and Granlund ($5 million) were all duds and making a huge amount of money. They only young players they had pushing for NHL minutes were O'Connor, Joseph, Smith and Poulin, and Smith was a complete bust and Poulin missed significant time in 2023-2024 due to mental health issues.
Dubas came in and made things worse with the Jarry and Graves deal along with the Karlsson trade, but the situation he inherited was a downright terrible one. There was not an easy fix to that 2023 Penguins team because they had about $18 million in unmovable dead money, an aging core and no prospects coming up. I think saying "he failed this easy task" is being disingenuous about how bad their situation was when he came in.