Yes and no. SSI is a term used by some companies to describe software that enables (i) organisations to issue one or more ‘verified credentials’ to an individual; and (ii) the individual to control who should see such credentials.
Here the term ‘ verified credential’ means much the same as what UCDx calls ‘trustworthy personal data’: both refer to personal data which the individual controls, in the sense of deciding who gets to see it, but cannot change. And both terms are general, and can refer to any kind of data, whether legal identity, or a qualification, or simply a statement that someone is over a certain age.
UCD differs from SSI is that it is aspires to be a complete proposal, putting forward not just clever software but also an organisational model, business model, funding model, governance model, and application route map, starting in UK education.