Nick and I chatted about this in Matrix and, yes it was due to conditional moz.build processing.
Bug 1570400 Comment 5 Edit History
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Nick and I chatted about this in Matrix and, yes it was due to conditional moz.build processing. To work around similar problems in the past, we've included the manifests from here: https://searchfox.org/mozilla-central/rev/d7d2cc647772de15c4c5aa47f74d25d0e379e404/toolkit/toolkit.mozbuild#9