Thats not rex, it may look similar but the events of shadow mosses took place in 2005 where as TPP is in 84, and with MG1 and MG2 yet to happen we still have a few models of metal gear even before that. Including metal gear D, and the TX-55. Kojima could have either reconed them, or had them fully redesigned, especially with the tech we now know they had around during peace walker. So it might be another transition model like zeke was
----
As far as the naked snake or solid snake debate goes, ive said my peace many times. And that i would rather have naked be the default this time, since he brings leagues more to the table while still remaining the character of snake. And most of all for the codecs it would feel a bit weird for solid to get intel again on the same characters hes already battled, which is why along with kaz, chico (who is also voiced by anthony del rio like pit) and many more, this snake would have a new cast and reason to learn about all the past characters with a new perspective and new personalities to reflect on each character.