You are right. A display method doesn't have capability to filter. Let's say if you need to get Person type vendors the developer needs to add a outer join with DirPerson and set it as a field in list page. Then you will get filter capability. But again organisation type is stored in different table. So again you need 2 fields for retrieving one party type.
In other case you can add a filter in the top of list page and give these lookup either person or organisation and then filter the below listpage. I would go with 2nd solution.
You can do it through customization by creating a new Display method and use it on the list page, but keep in mind that will affect the performance of the list page.
Thanks,
Waed Ayyad
If this helped, please mark it as "Verified" for others facing the same issue
Its a handled in code, you would need a developer to add it to the form as a display method:
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