Here Oliver Cowdery initially wrote had spoken in 𝓟; then he immediately crossed out the spoken and wrote inline testified. Thus 𝓞 must have read had testified. Note, in particular, the similar language in Alma 8:25: “yea and to testify against them concerning their iniquities”. Since either spoken or testified will work here in Mosiah 17:2, there would have been no motivation to emend spoken to testified except that 𝓞 read testified.
Summary: Accept Oliver Cowdery’s immediate correction in Mosiah 17:2: “concerning the iniquity which Abinadi had testified against them”.