The unicode characters use more than a single byte
q)s:"tést" / Store text with unicode to variable
q)s / Unicode bytes displayed
"t\303\251st"
q)count s / 2 bytes are used by é
5
q)-1 s; /Print to standard out presents the unicode characters again.
tést
Indeed the doc is inaccurate in that it's not the byte or character type itself that can hold unicode characters, but the lists of these types can by virtue of having a character span multiple elements of the list.
In this sense, `char` is very much like the C or C++ interpretation of "char". In that it is a single byte. It may contain a valid ASCII value, or it may be part of a codepoint of some larger UTF-8 sequence of bytes.