"Nibble" may well have always been in use by folks, and nybble may have actually come later. At the very least, references to each spelling being in use exist for the last ~60 years.

The first book match I get for "nibble" near "byte" is in the 1964 "System 360 Assembler Language" by Don H. Stabley uses nibble. The earliest match I can find for "nybble" in relation to computers was the 1968 "Encyclopedia of library and information science". Nybble (and likely nibble) itself doesn't seem to have taken off until around the mid 1970s https://books.google.com/ngrams/graph?content=nybble&year_st...

References to the coining of the term in 1958 of course don't provide a textual source.

I was wondering this as well. Probably when a new wave of people discovered the concept in the absence of the older wave? By contrast, "byte" has been in use continuously and widely.