Can I make biblatex refrain from assuming my postnote (which consists of a number) is a page number?

Add the prefix \nopp to the postnote: \cite[\nopp 13.9]{citekey}, or set the pagination field in citekey's entry in the .bib file to none

See section 3.13.3 of the biblatex manual.

From the BibLaTeX manual, section 3.13.3

It is possible to suppress the prefix on a per-entry basis by setting the pagination field of an entry to ‘none’, see § 2.3.10 for details. If you do not want any prefixes at all or prefer to type them manually, you can also disable the entire mechanism in the document preamble or the configuration file as follows: \DeclareFieldFormat{postnote}{#1}

On the other hand, when your numbers refer to sections instead of page number, it may be better to use \cite[section 13.9]{citekey} anyways, to make this clear to the reader, which will automatically suppress any prefix and suffix in the postnote field.