No, it says exactly that.
It is very different than saying “If a law restricting a right is effective, it is Constitutional”, which is not the same as “to restrict a right, it must be effective” (one states a sufficient condition, the other a necessary one.) Nevertheless,being effective is a relevant differentiator between one proposed restriction and another that explains why they might be seen differently Constitutionally, as while a law which is effective at serving some government interest may be unconstitutional, one which is not is almost by definition is not narrowly tailored to serve a compelling government interest.
But the comment I was replying to made a very different and I think highly indefensible argument: That the difference between red flag laws and no fly lists was the former was more effective.
It's pretty clearly the case that a process where the individual is not involved up front but retains the ability to challenge an unfavorable outcome can, for some things, be the process that is Constitutionally due. It's less clear exactly what the boundaries are for that, because the Constitution doesn't really explicit provide rules for determining what process is due for any particular action, leaving it in practice for courts to evolve standards.
The constitutionality of a law does not hinge on it's effectiveness.
I haven't formed an opinion as to whether red flag laws are constitutional or whether they are or are not similar to watch lists, but that's a terrible reason to suggest they are dissimilar.