@Tom R I was wondering if there is a way to update a runtime to 64 bit, without having the source code? Long shot, I know....
There's a difference between compiled and runtime based languages. Runtime makes it easier for the tool vendor to 'port' applications made with their tools to other platforms much easier; however this comes at a cost whereby the applications are slower (because they're converted from a human readable language to machine code on the fly), it also severally cripples their capabilities. I've worked with some developers who use competing tools to Xojo and none of them can ship applications on the Mac App Store as none of the runtimes support the requirements of it.
Not to mention that in runtime based languages, the original source code tends to be human readable still, meaning that anyone looking to make a living from software development can have their application easily duplicated and rebranded.
@Tom R The application is 32 bit, and Apple is soon discontinuing support of 32 bit apps.
Apple have unofficially discontinued support for 32-Bit Carbon apps since macOS 10.10. 10.13 is said to be the last macOS version that would run 32-Bit applications without compromises (whatever that means).
I do wonder if you could use a VM to run an older version of the macOS and therefor still retain 32-Bit compatibility.