References must be cited vs. need not be cited
This sentence is present on page 5 in the IEEE guide at:

https://www.ieee.org/content/dam/ieee-org/ieee/web/org/conferences/style_references_manual.pdf

The guide gives an explanation to it as follows:

References in Text: References need not be cited in the text. When they are, they appear on the line, in square brackets, inside the punctuation. Grammatically, they may be treated as if they were footnote numbers, e.g., as shown by Brown [4], [5]; as mentioned earlier [2], [4]–[7 ], [9]; Smith [4] and Brown and Jones [5]; Wood et al.[7]

This clearly means that numbers rather than the author name (s) are preferred to be used in the text. When author names are used, the guide gives some examples on how to cite them.