When the Islamic State claims New York City vehicle attacker, Sayfullo Saipov, is a "soldier of the Caliphate," it actually means that Saipov is "a useful tool."
The "soldier" descriptor doesn't mean that Saipov was directed by an ISIS operative, but rather that he pledged allegiance to the organization and acted in its service.
Don't get me wrong, while Saipov might have been trained and/or directed by an operative (inspired fighters often have some kind of basic communication with the group), ISIS takes a deliberately broad approach identifying its followers. It does so because it puts primary importance on owning acts of terrorism.
This separates ISIS from other groups like al Qaeda, who tend to ascribe membership or "soldier" status only to trained and trusted recruits. Conversely, as I noted in the aftermath of Omar Mateen's 2016 Orlando nightclub terror attack, ISIS has interest in recruiting even the most socially disaffected or mentally unstable individuals.
For ISIS, the key is an entrepreneurial dynamic in which prospective fighters will take up arms wherever they are. While ISIS' early focus was on recruiting young men to come to its self-described caliphate in Iraq and Syria, the organization now simply encourages violent acts in its name.
That speaks to the ultimate concern here.
While ISIS is losing territory and power, the group believes that committing global attacks will ensure it remains relevant. In the end, while ISIS leaders are on a theologically ordained mission to purify the Earth of secularism, they also believe time is on their side. In turn, even as the group grows physically weaker, it hopes that attacks such as Saipov's will ensure it remains a focal point for jihadist fundraising, recruitment, and inspiration.
Put simply, Saipov is a useful tool. Nothing more.