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Why would it not?

All that abhor the witch says is that you can't take any psykers, except for grey knights. It does not mean you take grey knight librarians or anything of that sort, but I believe the fluff counted all grey knight space marines as "minor psykers" (back in the day) thus the rule had to allow for them else they couldn't be fielded with grey knights at all.

Basically, it just means no one with psychic powers can be in an army allied with BT.

The converse would be true as well, where you couldn't take BT as allies if you have a psyker in your army.