For some context, the Massachusetts state government is helping the migrants flown to Martha's Vineyard sue the airline company that flew them out there (and I think DeSantis too?). I'm assuming that those who are involved in the case are the ones who are eligible for these visas, while those who may have chosen to not pursue legal action aren't. This doesn't necessarily mean that the others haven't gotten other forms of visas or anything though. It is Massachusetts after all, and Mass is a sanctuary state on top of Cape Cod having a major seasonal migrant workforce annually for the summer. The article also says that they had legal status in the US as asylum seekers.
Some further context from the article:
Now, some of the migrants have been granted U visas, which are “set aside for victims of certain crimes who have suffered mental or physical abuse and are helpful to law enforcement or government officials in the investigation or prosecution of criminal activity,” according to U.S. Citizenship and Immigration Services. They can eventually lead to permanent lawful status in the United States.
The migrants — who were from Venezuela and Peru — were eligible to apply for the visas after Bexar County Sheriff Javier Salazar, a Democrat, certified that they were victims of a crime.
After waiting for more than a year, some migrants earlier this month received “bona fide determinations” in their U visa petition, a status that allows them to obtain temporary work permits and protects them from deportation until their visa becomes available, Self said.
It was not immediately clear how many migrants had received that status from the federal government.
Congress only allows the federal government to issue 10,000 such visas each year. The “bona fide determination” gives the migrants targeted by the state of Florida temporary protections from deportation while they wait for the visa.
It looks like they said "some" because it wasn't said whether or not all of the migrants had received these visas in the announcement from the federal government.