Maybe what they should have added is "within an acceptable time period".
If it takes a skilled, determined manager (say) 6 months to mould a C player into an A player, then that's a big hit on the manager's time, and an eternity (in the modern fast paced world) before the employee starts delivering the goods. It might not make business sense to make that kind of investment, especially if the typical employee leaves after 2 years.
Sure, if your hypothetical determined manager never expects to need a job elsewhere, is never in a position where a previous employee speaking well of them is useful, doesn't need to hone their mentoring skills and never expects to be in a position where hiring someone who left years ago might be possible.
If it takes a skilled, determined manager (say) 6 months to mould a C player into an A player, then that's a big hit on the manager's time, and an eternity (in the modern fast paced world) before the employee starts delivering the goods. It might not make business sense to make that kind of investment, especially if the typical employee leaves after 2 years.