Cleanthes fails to grasp the strength of this rebuttal and is unfazed by Philo's demonstration. In fact, he sees in it a concession: at least Philo is finally admitting that the universe is obviously designed. Of course, Philo is not admitting this; he is merely arguing that even if we could infer a designer, we could not infer the sort of designer that the theist wants to infer.
It is important to realize that Philo is not trying to claim in this section that Cleanthes's argument from design actually leads one necessarily to these heretical positions. He is only claiming that Cleanthes's argument does not provide any grounds on which to deny these positions. This is a severe problem for Cleanthes. His claim (the standard claim of empirical theism) is that we can come to know all about God from the evidence provided by the natural world. Philo shows, however, is that it is impossible to use the evidence provided by the natural world to come to any substantive conclusions at all about God's nature. The evidence as well supports the claim that God is anthropomorphic, simple, eternal, and incorporeal as it does the claim that God might be actually be a team of mortal and muscle-bound workmen who copied a great designer and created a barely adequate universe.
As always, it is helpful to compare this type of reasoning to a more familiar example from everyday life. Imagine that you see a man bench press 400 pounds. You could certainly conclude that this man is very strong. But you could not conclude that he is infinitely strong, or that there is nothing he cannot lift. All you can conclude, given the evidence, is that he is strong enough to bench press 400 pounds. When we reason from effects to causes, we are only justified in inferring what is necessary to explain the effect. All that is necessary to explain the effect in the case of the man is that he is strong enough to lift 400 pounds. All that is necessary to explain the effects in the case of the designed universe (assuming, for the sake of argument, that the universe was even designed) is that there be some intelligent designer or designers behind it. Any further conclusions are pure conjecture, with no basis in any evidence.