Namco literally made Star Fox Assault, so it's likely they still have access to the source code, engine, assets, etc. and coud more easily rework those into HD and make tweaks to the gameplay. Nintendo remasters like Xenoblade Chronicles DE and Kirby: Return to Dreamland DX show that it's possible for them to go back and not just spruce up the graphics to look nice in the HD era, but to also reuse the assets to create new single player content, and introduce online leaderboard elements in Kirby's case.
Sure, they could spend a lot of time reworking Zero from the ground up, but why go to Namco and not Platinum,
who publicly asked to work on it?
IF this HD remaster is Star Fox-related, it would make more sense for Bamco to do Assault, since they made it themselves and therefore have access to the materials and possibly even some of the original staff. It's as simple as that.