This phrase comes from the obituary notice of Miss M.L. Wood (the so-called “Mother of Manx Music”):
“Though not Manx by birth Miss Wood was essentially Manx in sentiment, and all her compositions are influenced by that sentiment.”
Anon. “Death of M.L. Wood.” Peel City Guardian 10 January 1925:  col. d.
As we know, the notion of just what or who for that matter is “Manx” is a constant Island theme and was seen in the conflicted view of whether TEB was Manx or not despite having in his case been actually born in the Island.
It would be interesting to gather together other views and opinions on this topic.