You are half right, it is developed on an "over-specification hardware" and will be optimized for the PS4. But given the sales and the crowd, the Playstation is still the main cash cow for the series.
The reason behind my question is kinda simple: sales on the xbone* VS the sales from the appeal of a PS4 exclusive. That appeal worked for MGS 4.
*very low xbox sales for XIII
Low cost/easy port = easy profit
Gotta understand that if both consoles can run Direct X 11 then we are talking about ports that would take a short period of time to move (build) on the consoles.
If that's the case why wouldn't you? Would it be more prudent to ask Sony to co-market in exchange for time exclusivity? Probably. That's an avenue they should have taken if they didn't consider it. But in any case not porting it to another console just cause is dumb. Especially when it takes a short period of time and money to do so. Even if they don't sell a lot on Xbox One the return on investment on that port will nullify even if it only sells a few hundred thousand.
Edit:
Trust me though I would much rather they had it as a PS4 exclusive. Those programmers, input designers and artists would be better served assisting making one amazing product instead of splitting up resources to make sure both are great. But it shouldn't affect development all that much.
Now if we were talking a Wii U port, that would divert way to much attention from the team to do the port and would negatively affect the game. Just be glad they're not doing that.