unregistered dealer can make interstate exempt sale

Yes.

Sec. 24(1)(i) of CGST Act says that, persons making any inter-State taxable supply shall be required to obtain registration compulsorily.

But the supply should be taxable. So, exempt supplies can be made.

Further, if you are exclusively dealing in the business of supplying goods or services or both that are not liable to tax or wholly exempt from tax under CGST or IGST Act, then you are not liable for registration, even if such goods/services turnover crosses 20 lakhs. Sec. 23(1)(a) of CGST Act.