with the so called "Ultrasonic Welding", you do move something very rapidly against the other surface to create HEAT and weld the two pieces together. Usually you do produce a lot less heat than with something like a spot welder but you do have to move/vibrate the wire and/or the battery (60K hertz I read above) and you can't move one part against the other without transmitting some vibration to the battery
60K hertz is 60 kilohertz, or...60,000 cycles per second. Its not so much the fact that there is vibration occurring, it is the distance that the two components are moving. The faster you vibrate the two parts, the less distance they need to move in order to achieve enough friction in order to create enough heat to weld the two parts together.
The amount of milli-seconds of vibration needed is very adjustable so that anyone who spends the money to buy the equipment that does this, can be assured that there is a very solid electrical connection, and the amount of heat generated is very localized and focused so that absolutely no heat reaches the inside of the 18650 cell.
As to vibration penetrating the cell, the mass of the connecting wire that is vibrated is very small, especially in comparison to the cell it is being attached to. Therefore, the cell experiences no measurable vibration. The mass of the cell has no time to react to the small wire vibrating a tiny distance about 60,000 cycles per second.
If any vibration ever did penetrate the cell, I am unsure if that would cause any damage, but...just to be on the safe side (because I don't know everything) Luna has thoroughly tested the prototype packs made this way. Heat, cold, extreme moisture, drop-testing, vibratory cycling, etc...It's my understanding that they followed the wire-bonding protocols used by Tesla (who has been very open about their patents), and it has worked out fairly good for them so far.