The module does not manipulate products info in any way. It does pass to Google on the order confirmation page a set to 'N' variable if order is on backorder but the value can be modified if the store needs it, it is store specific. As per Google Trusted Stores docs if even one item in an order is on backorder the whole order is treated as backorder
https://support.google.com/trustedstoresmerchant/answer/2609894?hl=en&ref_topic=2609888#Section2
Quote:
HAS_BACKORDER_PREORDER
Y if 1 or more items on the order is known to be on backorder or preorder. Otherwise N.
Required
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