Enabling the Disable GCP Non-Project Cost Reallocation feature will ensure Unidentified Project Costs do not get reallocated to the projects but this setting does not apply to historical dates automatically unless bill collection happens again for a historical date due to cost change.
For example, if the Disable GCP Non-Project Cost Reallocation feature has been enabled on May 16th, only the daily cost going forward will match up as expected. Project level cost will only match up for May if costs between May 1st - 15th are recollected due to a cost change.