From the article, once the tumor is broken up the immune system begins to clean up the area and is able to learn antigens against the tumor. So your immune system would attack cells that are spreading around the body.

I think that would be a very real concern that will need to be evaluated post approval.

The approval was based on the device efficacy as a surgical tool for removing tumors, not as a treatment for cancer. Because of this, the trials did not look at long term outcomes like if the procedure leads to the cancer recurrence or spread.

In practice, The device will likely be used in conjunction with chemo, to try to hunt down metastatic cells.

I'm no expert either, but there seems to be a nonzero chance of that. However, according to the article, it is offset by the immune system learning of the contents of the destroyed cells and attacking these even elsewhere in the body.

In theory - maybe, sort of.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5380551/

Ultrasound with a focus lens destroys the cells by creating cavitation (implosion 'bubbles') it does not move the tissue somewhere else.

It's basically the same as tumour tissue removal but without invasive surgery.