A tube is strong
Engineers use tubes because most of a solid beam’s strength is in its outer part. A tree works the same way. As a yew’s heartwood decays, the living tree keeps adding wood on the outside, and a thick, even wall can stand for centuries.
That’s why so many ancient churchyard yews are hollow, and why a hollow on its own is not a reason to remove one.
What we look at instead
We look at the wall: how thick it is, whether it is even all the way round, and whether there is an opening. Mattheck’s well-known guide, a residual wall of about 0.3 of the radius, was worked out for closed hollows; with an open side, the wall and the opening have to be judged directly.
A tomogram helps here because it shows the wall as a ring of sound wood around the void, with the opening as a gap you can measure.
The count you can’t make
Ring counts on hollow trees are always minimums. The rings that decayed are gone, so the honest answer to “how old is it?” is “older than we can count.”