Can my exlandlord tell people I was evicted if he agreed to have the eviction sealed by the court?
Yes. You would need an agreement for confidentiality to stop the landlord from disclosing your eviction. Sealing the record only stops people from going to the court record and seeing your file.
On the other hand, if you want, you could go to court and seek an order of confidentiality, on the theory that the landlord's agreement to seal the record was tantamount to a confidentiality agreement. So the Court should order confidentiality. This route would be longer and could be expensive, but, if it's important to you, you might give it a try.