Yes. Bernardo is making the point that Witness 8 is so scared of becoming known, that she identified using a nickname. He goes on, "See attached letter."
-- Another thing I find odd: why would W8 close a note like this with "Thank You"? --
All Bernardo said was that Witness 8 gave the handwritten material to Sybrina. Bernardo is inviting you to conclude that the writing is a letter. It obviously isn't a letter, by its contents. I think it is a something in the nature of a cheat sheet or memory enhancer, handed to Sybrina, maybe intended to be delivered or shown to Crump before that first telephone interview. The "Thank you" is to either remind herself to thank Crump (for the opportunity to speak out), or for Crump to thank her for coming forward.
A re-read of the Response confirms you are correct — he does claim the note is signed with a nickname to hide her identity. And BdlR is such a standup guy, he even blacks out fake names.
I speculate that W8 is thanking Sybrina for helping her develop the talking points so she would know what to say to a big scary lawyer, and for solving her desire to stay out of public involvement by arranging the phone interview scheme (oops...jokes on her). None of that need be nefarious on W8’s part, but would seem to raise enough reasonable doubt to sink W8’s testimony.