DIY usb power booster

That is super terrible design... but will it break anything? Good question..

If you try to backfeed a voltage into a supplied voltage, the difference in the two will determine the amount of current that flows. So if the supplied current is 5.1V and the applied voltage is 5.2V, 0.1V will cause current to flow into the 5.1V supply, unless it's specifically designed to prevent current from flowing backward. (this would depend on how the switching converter is designed)

So say you have a long cable, and 0.1V of mismatch... that might be around 5 ohms... That is 0.020A, or 20mA. That is enough current to run an LED, but usually not enough to do damage to a power circuit. A much worse condition (as Russell points out) is when the computer is OFF and there is power being backfed.

Most USB power systems are going to have a resettable fuse somewhere around 500mA to 1.5A, so that would protect things in an extreme case, but the real answer is that it depends on the computer, and it depends on the design.

My best advice is to stay away from USB hubs that are of mindblowingly poor design.

The datasheet for the TPS2115A you're considering lists its ESD ratings as 500V and 2kV (depending on the model) in the same 'Absolute Maximum Ratings' table which you're seeing the 6V limit.

I read that as 6V being the maximum sustained operating voltage (although proper operation above 5.5V is not implied).

While your teammate is correct in in his position that a 5V TVS device will not prevent the input voltage from rising above 6V during an ESD event, I believe that he is incorrect in saying that any ESD will automatically damage the device.
According to the datasheet, you don't need to ensure that an ESD event doesn't raise the input voltage above 6V - you rather need to ensure that it doesn't exceed the 500V or 2kV limits.
How you do that (filtering, TVS's, whatever) is another question.

Choosing a different device (with a higher voltage rating) does not give you any automatic assurance of ESD protection.