In the Outlier entry there is a link to a gloss explaining the term:
Sound Loan is when a character gets borrowed to represent a word for its sound, rather than its form.
Ex. The form 我 wǒ which today means "I; me" was originally a picture of a type of weapon that consisted of a handle and a blade with saw-like teeth. The name of the weapon sounded similar to the word for "I; me," so it was borrowed to represent that word. Eventually, the meaning "I; me" took over completely and the form 我 was no longer used to represent the name of the weapon.
Ex. The form 其 that now represents the word qí "his/her/its" originally depicted a basket. That word meaning "basket" is now written 箕 jī.
In Outlier you will often see the ○ icon in the meaning trees to indicate a phonetic loan. Which are generally defined as a:
character that is "borrowed" to write another homophonous or near-homophonous morpheme
So, yes, your assumption was correct - “to seek, request” originally existed as qiú without a specific character associated with it. The character「求」was then later borrowed as the readings matched.