Canon 87 states that the bishop of his diocese is competent to dispense from specific liturgical laws enacted by the pope if it is for the spiritual good of his people. In this case, several American bishops, including my own here in Oklahoma, have determined that allowing the Latin Mass and the other sacraments to be done in the old rite is a positive spiritual good in their diocese, so they permit their continuance despite the motu proprio Traditionis Custodes which restricted them generally.
Cardinal Roche’s letter is now saying that these bishops are mistaken and do not have that authority to so dispense under Canon 87. These bishops are now considering whether to concede the point or to fight it under canon law.
Hope this helps make it more clear.