The problem is it doesn't actually say that - the above reads like an interpretation from a training provider!
"15. —(1) The responsible person must—
(a) establish and, where necessary, give effect to appropriate procedures, including safety drills, to be followed in the event of serious and imminent danger to relevant persons;
(b) nominate a sufficient number of competent persons to implement those procedures in so far as they relate to the evacuation of relevant persons from the premises;.........
...... (3) A person is to be regarded as competent for the purposes of paragraph (1) where he has sufficient training and experience or knowledge and other qualities to enable him properly to implement the evacuation procedures referred to in that paragraph."
Now in many places compliance with the above will indeed be demonstrated by having lots of named wardens trained annually,but it is not automatically correct for everywhere - some places will require an even more rigorous regime of staffing & training, others can safely have less frequent retraining (with drills in between) & in yet other places named wardens are not practicable