Yes, exactly. My attestation letter was NOT in a sealed envelope, but the letter itself did have an embossed stamp (also known as a seal) on it. This kind of seal is what is required. I do not think that a sealed envelope is what CIC is requesting.
And for those who said they submitted a transcript instead of the attestation letter because the checklist said "Attestation Letter OR Transcript", your application will probably be returned to you if you have not completed your PhD. If you look at the details in the checklist, the option of sending only a transcript and not the attestation letter is only available to those who have finished their degree and graduated. If you are still in your PhD program, you MUST include the attestation letter. Transcript should be included, but the letter is a necessity.