I didn't say that sick leave is not for employees who are not under ESI only, but if your Shops Act states that an employee is entitled to 12 days of sick leave, that applies to employees covered under ESI as well.
For example, an employee falls sick while on vacation and cannot be admitted to an ESI hospital due to various reasons but undergoes treatment at a private hospital. In this circumstance, he can take sick leave even though he is covered under ESI. If he submits a sickness report from ESIC, it will be considered as ESI leave for which we will not provide salary, but ESIC will compensate for it. If he does not submit the sickness report but provides a medical certificate from a different medical practitioner, then he will be granted sick leave.