Salamun Alaikum.
'Fahm' as used in 21:79 is a term that refers to an understanding or knowing of a thing. So 'fahm' can be a component of wisdom (hikmah) and so there remains some linkage between the two.
However, just because one has understanding of certain matters, doesn't mean they have 'hikmah' which normally appeals to a wider, intuitive ability to be sound in judgment and be able to discern and interpret the knowledge one has.
Note in Surah Luqman (31), how Luqman uses his wisdom (31:12) whilst imparting his knowledge to his son. For example:
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"O my son!" (said Luqman), "If there be (but) the weight of a mustard-seed and it were (hidden) in a rock, or (anywhere) in the heavens or on earth, Allah will bring it forth: for Allah understands the finest mysteries, (and) is well-acquainted (with them).
Note how Luqman translates knowledge of God's immense power, authority, intricate absolute knowledge and governance into a simple manner that his son could understand. That is wisdom (31:12)
I hope that helps, God willing.
Joseph.