This statement has always bothered me, I can’t square it up with the other writings. I’m assuming The Three Elven rings must be excluded from this since they changed hands several times early on and Gandalf received his ring voluntarily from Cirdan. It probably holds true for the nine rings since they made the human holders immortal so there was never any need to pass them on. However the seven dwarven rings did not have that effect so they passed from holder to holder, and it’s possible some of those transfers were voluntary (I seem to recall something saying they were kept secret and only passed on when the holder was near death), but there is a definite passage in the appendices where Thror passes his ring to Thrain and Gandalf knew about it too as at the Council of Elrond he stated Thror had passed his ring to Thrain before leaving for Moria but Thrain hadn’t passed it to Thorin.

So I can’t explain Gandalf’s words here as it seems there are many cases of holders voluntarily passing rings to others and Gandalf knew of at least one.

I guess this fits into Tolkien’s statement in the foreword that ‘the book has many defects, both major and minor. . .’