I want to automate the import in magento 2.
How can I call vendor/magento/module-import-export/Controller/Adminhtml/Import/Validate.php via cron?
You shouldn't be able to do that becase the adminhtml is protected (ACL rules, authtentication) and cron jobs runs in a different area (frontend, adminthml, cron).
I guess you want to get this part of the controller into your cronjob:
You should check how to be able to execute the validation on your job.
(my 2 cents)