The point being made is...
... that the employee is now paing tax at 20% but expects to be paying 42% on his/her retirement income.
If this is the case the argument is quite valid and perhaps AVC payments should be held back until they are paying at the higher rate.
However the converse is true and probably applies to most people. i.e. that those that pay AVCs will in fact be paying less tax in retirement as the pension that most can fund for after the lump sum is taken is a pension of approx 50% of final salary. Therefore the majority of people in retirement pay no tax or tax at 20%. For those people AVCs will make sense.
There is also merit in investing in AVCs where the tax treatment is similar in retirement.