Merge PowerPoint presentations with Javascript Automation

The goal is to do a 3-way merge with git using PowerPoint's mergeWithBaseline method. I can load and access a presentation with the following:

PPT = Application("Microsoft PowerPoint")
original = Path(pathToMainFile)
PPT.open(original)
PPT.activate()
PPT.activeWindow.presentation.mergePresentation({withRevisionPath: pathToRevisedFile})

However, that last line using mergePresentation doesn't appear to do anything. If I do it manually in PowerPoint (Review->Compare), then PPT.activeWindow.presentation.inMergeMode() returns true.

Hopefully someone with experience in Automator can explain why that merge operation is working?