Yes. It is still possible to have a seamless integration even it does not have an open API. There are four alternatives for data integrations in Salesforce:
Even if REST and SOAP API don’t work accordingly, solutions like APPSeCONNECT bridge that gap for efficient data integration.
Our integrations are dynamic in nature. The mentioned offerings are the cumulative possibilities of the platform (actual scenarios are specifically organization/business-dependent). For any disparity, please refer to the latest version of the integration brochures.