A 'trunk' or base like that would take a long time to achieve so yes, probably quite a venerable shrub. Could you grow one like that? It is certainly possible, but would take a lot of time and patience. Rather than limiting growth to a single stem as is typical with tree-form shrubs, it appears several, quite straight stems were encouraged to grow vertically. it is difficult to determine from the photo if any inosculation or actual melding of the stems has taken place and that would not be necessary to achieve that result, although it could happen.

Even at the obvious maturity of this plant, attention will be required - one can very clearly see green shoots appearing along the trunk/base and these will need to be removed promptly to maintain the tree form.

Yeah, that shrub has been around several decades! Reminds me of some of the Crape Myrtles in my neighborhood that date to the 1970s.

I wonder if Hydrangeas can be fusion-trunked like trident maples on this webpage https://adamaskwhy.com/2015/03/03/fusing-trident-maple-trunks/ Trident maples are unfortunately not hardy in my area, whereas Hydrangeas are. I would naturally start with a shrub with a dense multi-stem structure in the clump.