Thomas Tempelmann has linked to the article https://github.com/microsoft/appcenter-sdk-apple/issues/2135 which points out that this a false positive from Apple. Thomas also links to Apple's documentation at https://opensource.apple.com/source/cctools/cctools-949.0.1/include/mach-o/getsect.h.auto.html which proves this to an incorrect rejection reason.
Since then several developers have reported success in submitting their apps, Xojo have also said that this will be corrected in a future build.
Several users have posted in the Xojo forum https://forum.xojo.com/61457-mas-app-rejected-because-uses-non-public-apis-symbols-getsectio about having their latest Mac App rejected by Apple.
Together we've narrowed it down to the Xojo shared library that causing this issue. Some research into the function name indicates that it's used to load data from the Application executable (Which is a Mach-O file), and it appears that this function has been used by Malware.
This affects Xojo 2018r3 ~ Xojo 2019r3.2. I have not tested any prior versions as I currently do not have those installed on my work Mac.
There are two possibilities I see.
At the moment, there's not much we can do, I can patch that function today, but I'm not willing to do so, until I know what the outcome will be. Depending on how long it takes to fix this problem, I may at the very least attempt it and suffer the consequences.