UA's specification for pet carriers says:
The recommended maximum dimensions for soft-sided kennels are 18 inches long x 11 inches wide x 11 inches high (46 cm x 28 cm x 28 cm). Soft-sided pet carriers may exceed these dimensions slightly, as they are collapsible and able to conform to under-seat space without blocking the aisle.
This picture of the dog carrier in question looks the right size to me.
If it didn't fit, there's presumably something wrong with UA's measurements. Or is the 'fits under the seat' assessment left to individual FAs as a judgement call?
I can see that this bag looks similar to normal luggage, but surely FAs are given some instruction on what pet carriers look like?