Here the 1840 typesetter accidentally replaced knoweth with knowest, an obvious typo. No subsequent edition has continued or reintroduced this error. For further discussion of mix-ups between the archaic inflectional endings -eth and -est, see under 1 Nephi 11:2.
Summary: Maintain in Alma 26:21 knoweth with its original inflectional ending -eth (“what natural man is there that knoweth these things”).