I think some are reading too much into the extension, I don't think it necessarily says anything about how management view the player long term. It's simply a prove it deal, I believe management rate him highly and see him as a piece long term but the full results are not quite there yet.
The biggest thing he needs to work on is his decision making defensively. While he is a strong skater he sometimes was beaten due to slow reactions. It seems he's slimmed down a bit this summer so hopefully that may help some but it will hopefully also just improve with more experience and time which this contract allows for.
As others have mentioned it takes time for D, especially bigger D like him to adapt to the pace. Xhekaj is one that I think we need to just let cook over the next 3 to even 5 years that I'm certain will pay dividends come his mid to late 20s.