According to
The Lord of the Rings, first and second editions, the Silvan Elves of Mirkwood and Lorien were not Eldarin, nor their languages Eldarin (Appendix F).* Also according to
The Lord of the Rings, the Eldar are those Elves that crossed the Sea plus the Sindar only.
Thus for me, Mithrellas is:
A) not Eldarin (not a "High Elf" first edition**)
B) part of a legend in any case
And Tolkien's footnote to the second edition about Sindarin being spoken in Lorien can, in my opinion, be taken to mean that
some Elves also spoke Sindarin in Lorien, but with an accent.
In my opinion a measure of oversight and changing conceptions are involved here, but for me, what matters in the end is the picture that Tolkien himself drew. For some reason, Tolkien's
posthumously published ideas about the Nandor being "Eldar"
seem to have overshadowed Tolkien's published conception of what the term Eldar referred to . . .
. . . but not for me
__________
*In
draft texts Tolkien employs the term
Avari but does not ultimately publish it, opting for "West-Elves" (Eldar), and "East-elves" (which need not mean Avari).
**The union of Eldar and Edain is a revision from the first edition, which had "High Elves and Men" rather.