why xps 13 9350 cannot have a replacement for battery swollen?

The difference is that the 9550 is seeing premature battery failure.  The 9350 failures are due to normal aging and end of life -- a lithium ion battery will run about 18-24 months in a normal lifespan -- and the 9350 systems are of that age or older.  In other words, the batteries are due for replacement.  

even if the battery is aged or run out of its life, it should NOT cause swollen.

it is normal if the capacity is decreased, but swollen is not.
The batteries swell is mostly like caused by overcharging, which is controlled by Dell's firmware.
That's why Dell should take responsibility to replace batteries and update their firmware.

https://www.quora.com/What-would-cause-a-phone-battery-to-bulge-in-the-middle
https://powercontinuity.co.uk/knowledge-base/why-do-batteries-swell/

even if the battery is aged or run out of its life, it should NOT cause swollen.

These batteries have no rigid shell around them.  When they reach end of life, they WILL swell up -- doesn't matter if it's a Mac, Dell - or any other ultralight system battery.  At the end of life, the electrolyte will break down and they'll swell.

It IS true that it's abormal for a battery under a year old to swell up -- but after 18-24 months, the batteries can swell not from a manufacturing defect -- but due to old age (just like any ordinary alkaline battery will eventually leak if it's left in a device when it's old and overdue for replacement).

I spoke by chat with Dell online support and they sent out a tech with a replacement keypad and battery at no charge.

You are so lucky. I called to Dell support but they reject my request and ask me to send a quotation.

18-24 months ? Are you on drugs ? That is a non answer.

With answers like I see here, first and last Dell anything for me.