FMI documentation is limited.
I would think that FMGo 13 is not supported because it would have came out before ios 9.3 and FMGo 14 was already released. FMI usually does not go back and update old versions of software even though it was still available on the App store. I would guess the newest version of FMGo 14 would be compatible because it was the current version when ios 9.3 was released. This would be my speculation from the documentation that you provided above and from using FMGo / Filemaker. My search results didn't find anything useful. FMGo 13 had a memory leak that caused crashes.
I had FM Go 11, 12 ,13 and 14 all installed on my iPhone 6, and they all continue to work in iOS 9.3.1, along with 15.
I recall there might have been a notice that FMGo 13 is going to be removed from iTunes in Q3 this year.
13 still seems to work for me on 9.3.1.
Yes, FMI does keep removing older versions on a schedule to encourage upgrades, but if you installed one while it was still available it keeps working after discontinued in the store, and, sometimes can even still be redownloaded from your Purchases screen if no longer on the device. (But No guarantee of that indefinitely.)
It looks like everyone has answered your question pretty well. FileMaker Go 13 is not Tested working in iOS 9.
For future reference (Which looks like you found -it is updated with the software, so iOS 7 and iOS 8 for Go 13 is accurate): FileMaker Go operating system requirements - all versions | FileMaker
I hope this helps!
Thanks for that document, for some reason I couldn't find it when I did a search.
On the device that I upgraded to 9.3 I am able to open (not tested beyond that) FMG 11, 12, 13, 14, and 15 which means I can support the current and future customers of the current version. (The best prospect for buying FileMaker 15 is someone who has not yet bought FileMaker 15, and yet is being well supported in the FM version they are stuck at for the moment.)