A little early in the season for an Appeals question - you took me quite by surprise!
If Kent Admissions say that it is a useful argument at Appeal, then who are we to argue?
However, please remember that there will be quite a number of others in the same situation. The only
primary grounds for an 11+ Appeal are that your child is academically suitable for a GS, and you must always be able to provide proof of this from their schoolwork or report.
Other "mitigating factors" - such as the way the test is administered are generally secondary arguments.
I agree with the implied comment in your post, that Kent Admissions should be trying to create a "level playing field" (I hate that expression!), but if people are coming long distances to sit the Kent test, or even travelling from abroad, then it really has to be one day of tests.