Future "Starting Validity Date"

A new employer Company filed H1B transfer and got approved but email say Starting Validity Date: 08/29/2021 todays date is 06/29/2021 .

Can I join new employer since I-797 got approved ?

what does it mean by Starting Validity Date: 08/29/2021 which is in future ?

What if I join new Employer now where the Starting Validity Date is 08/29/2021 and my current employer revokes H1b before 08/29/2021. ?

No, since your approval says ‘Starting validity date of 08/29/2021’, you may not start the job with new employer before 08/29/2021.
Check with your attorney who filed your change of employer/transfer petition.