I don't think "to them who" is "obviously wrong", but to me it feels mostly literary, or a tad old-fashioned-sounding (like something you might read in the Bible). It seems out of keeping here because it doesn't fit the style and content of the rest of the sentence.
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Hi, Mr Wordy
Thanks for your reply.
I just found something relevant here.
The principal reason for our failure was quite apparent to those/them whom we had
brought into the venture.
English requires that we use a demonstrative pronoun (those) to point to or determine the referent for the relative clause.