My inclination is that the footage is real, but my hope is that it's not because it was already bizarre that Vader did not appear with his ANH look in ROTS, and at least Rogue One brought it back, it would be bizarre to have him in an ESB look for this show because it would be like, why did he change his look briefly around the events of ANH, but before and after he had a different look. It feels natural if he had a look after he first put on the suit and then over the time between ANH and ESB, he evolved it.