This is just a confusion of English.
"Supplied" is a bit of a weird term to use, I agree, due to the direction of information flow, but feel free to substitute in just "connected to". It seems pretty common to use this word, for example Wikipedia says:
A dermatome is an area of skin that is mainly supplied by afferent nerve fibres from the dorsal root of any given spinal nerve
and I see similar phrases elsewhere (not sure which is your original source). Afferent nerve fibers, as you describe, are those carrying information into the central nervous system from the periphery, so yes, the information is coming in from the skin.
If you interpret it instead as the spinal nerve providing the connection rather than information flow, then it makes a bit more sense to say "supplied". I suspect the phrasing is borrowed from blood supply since nerves and major vessels tend to develop and travel together. I also suspect the phrasing comes from anatomy, which is sometimes not particularly interested in function which can lead to some confusing results. You'll also see similar phrasing in standard English in other contexts, like roads, even when movements of goods or people are two-way. Merriam-Webster also provides a definition addressing exactly this circumstance:
to furnish (organs, tissues, or cells) with a vital element (such as blood or nerve fibers)
So yes, a given spinal nerve is "supplying a single dermatome with nerve fibers", even if the information ultimately moves the other direction.