Once again, it all depends on how you define your terms. There isn't an "actual tree" in the sense of single stem you can point to. What you have is a spreading, generating system with above- and belowground parts. Yup, that genetic individual has been there for X amount of time, and their calculation of age from rate of spread assumes a lot, but is OK.
It's analogous to my colleagues publishing articles about the most massive living thing ever, a genetic individual of an Armillaria. So sure, it covered a lot of ground with a lot of mycelium but people wanted to see a mushroom the size of a skyscraper, and it just doesn't work that way.
I don't think it's much different from taking an amoeba that reproduces by division and saying that this amoeba is a billion years old because it has been dividing that long with no break in the continuity of life. If we reject spontaneous generation, then every thing alive now has been a continuous part of one living thing or another since the whole life thing got started. And you can define that last bit however you like!
Yes, yes, it is all connected and neat. It just shouldn't surprise anybody.