Sorry
That's Incorrect

No. We can live with the posttest-only aspect of this
design, but it is the inclusion of only a single group that
poses the big problem. With but one group we can
apply a treatment but will have no comparison, no way of
judging whether the response variable has changed over time,
or  if the results are different from results that would have
been obtained had the treatment  not been carried out.
The addition of a pretest would help to tell us if anything
changed, but it requires the addition of a second group,
a control group, for us to have confidence that the treatment
was, indeed, the effective factor.  Is all that clear?