Maybe Tepan don't know any evidence that endorses his way of using prepositions before "la", but I found those in the Official Toki Pona Book:
1. Sonja mentions "fragment" as A in "A la B." She doesn't explicit it must be a noun or a pronoun (she doesn't even distinguish nouns and pronouns). As a prepositional phrase is indeed a sentence fragment, it may go before "la".
2. In an exemple, Sonja uses "tan seme la" and translates it just as if it were "tan seme" at the end. Although she doesn't explicit the literal interpretation like in some previous examples, it's simpler to assume she meant "from what" than "what reason la", just changing the order of the sentence.
2.1. As a complement to the argument 2 it can be said that the nominal POV (that of the dictionary) of "tan" is a preposition, further validating that it should be read as such before "la" and also requiring another step of reasoning to get at the noun "reason" instead of "because of".
3. The definition given by Sonja for prepositions is that it introduces a noun. Nothing is said about connecting that noun to anything else. Prepositions can even be used without a verb.
I didn't add to those arguments that Sonja isn't prescriptivist, but rather describes her own way of using toki pona, implying freedom of interpretation of the rules, providing it keeps the simplicity and structure. Moreover, that's the way the community uses "la" and prepositions.