No. First, let us look at this little tidbit:
Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education...
So there was manipulation of the data to exclude the effect of these "other factors", which completely throws out any correlation that these could/would/should have. It would be akin to testing if teen pregnancy lowered IQ, but they threw out data belonging to private school girls.
This is a very common thing to do in medical studies. It's called correcting for confounding factors. If they hadn't done this then the results would have been less useful, as it could be claimed that the observed effects were the result of something else already known to negativly impact IQ.
they found that those who persistently used cannabis - smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s - suffered a decline in their IQ.
This is plain bad science. These people they are studying are CHRONIC users. They are likely using right up to the morning of their "interview". It is like the kid who started smoking cigarettes at 8 years old vs. someone starting at, say 23. The former is most likely to smoke 2+ packs a day. The latter usually smokes less than one pack. Also, nothing has been done to show what happens when they would stop.
First off, why is it "bad science" to test the effect on chronic users? As for what would happen when they stop, from the abstract:
Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users.