I kindly disagree with that.
And I encourage you to CodeSign (with Hardened Runtime) each and every DebugRun on macOS, allowing you to develop and debug as close to a Release Build as possible.
Well, I think Javier refers to the fact, that for debugging, the app gets the get-task-allow entitlement, which allows debuggers to connect, while the built app doesn’t get it.
Debug and built apps are both code signed.
I think I explained bad myself, sorry. I mean, currently we can’t attach the IDE debugger to macOS projects executed with Hardened Runtime enabled (they simply stall). If we find a way… we will add it to the mix without doubt.
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Ok. Maybe a language barrier 
Now I read this as “a Xojo IDE running on macOS can’t debug its built & launched (debug)app if it codesigns it with hardened runtime”.
And I don’t think this is a true fact, as I’m doing this all the time (codesign with an own post build script - with hardened runtime enabled). And I’m having no issues at all debugging from within the Xojo IDE the debug.app.
That’s either because a Xojo built debug.app has some debugging stub baked in (so it doesn’t need the get-task-allow entitlement), or it’s because that might only be a macOS ARM issue (I’m still using an Intel Mac). 
We have been adding a lot of stuff to this side of things lately (macOS code-signing related), so… what was not possible before maybe it could be for a next release. Let’s see. 