I was looking through this today and filtering for only external_tool submission types will return more results than just turnitin enabled assignments if your institution uses more than just one LTI as a source to submit to. In Canvas Data, we are missing a field titled external_tool_tag_attributes which would provide the URL for the external tool. If we were provided this, then you could filter off that URL if it contains "turnitin".
Since this is present currently only in the Canvas API, I have just created a feature request for the data team to add the external_tool_tag_attribute fields to Canvas Data.
TL;DR you can find turnitin enabled assignments through the API, but not through Canvas Data at this point 
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