If I understand dendrochronology, they are compiling--actual samples--a time schedule that will eventually be continuous all the way back, but that will vary from one region to another because climate varies from one region to another. Anatolia might have had a few good growing years while Attica was bone dry, for example, so samples could not in general be directly compared.
IIRC, that is the weakness of dendrochronology. If you do not have a means of connecting tree rings regionally, then you lose the continuum. The statistical confidence level drops and you can't piece together a reliable timeline.
Yes, regional differences do occur however, they are still useful for determining worldwide events.
"Dr Mike Baillie, Professor of Palaeoecology in the School of Archaeology and Palaeoecology at Queen's University, Belfast, Northern Ireland, is a leading expert in dendrology, or dating by means of tree-rings. In the 1980s, he was instrumental in building a year-by-year chronology of tree-ring growth reaching 7,400 years into the past."
I've read that the tree-ring records have been extended back to 10k years now.
One trouble is that (for an example from the paper) two trees growing on the same hillside but different elevations generally have quite different and unmatchable tree ring sequences, even when they are contemporaneous. There are also problems with soils, such that volcanic soils are enriched in C12, leading to plants showing false antiquity.