You'd be breeding one bird to another of a specific breed and that would be breeding, that's for sure. You still won't have a true heritage strain of that breed, though you will have a bird that is considered as stemming from a breed that was started long ago and is now called a heritage breed.
Now, if you bred those two and picked out the offspring that most resembles what the original breed looked and performed like, and then bred those birds and kept the offspring most like the standard, and kept doing that over and over down through the years until you had returned your original strain back into something that looked and performed like the original breed standard, you would then have a heritage quality bird.
If you just bred two RIR birds together and did nothing to try and sway those pairings to improve and move towards the original standard, then you'd just have a flock that may or may not resemble the original two birds. They would be red, they would lay eggs and would taste like chicken. Depends on if you have any pride or integrity towards the true heritage of the breed what you would call your birds at that point.
They still wouldn't resemble the original breed called RIR unless you had obtained those two birds from someone who was breeding towards that standard, had achieved some measure of success in that endeavor, and you too had continued their work towards that goal.
Some folks either don't know or don't care and they advertise their chicks as coming from a heritage breed flock, but when I ask about the source and they tell me they are just hatchery birds they bred together and are selling the chicks, then I give it a pass...this is someone who knows nothing about chickens and probably isn't developing his flock for the best genetics in the first place. He's calling them "heritage" as a selling point only...if not he'd just advertise RIR chicks and wouldn't put the word "heritage" in there at all.