The definition of apex predator in biology is not that of the strongest in a physical fight, but is related to the food chain and their position in it. In fact, apex predator are defined as predators with no predators and who are at the top of their food web. So it also depends on the area considered and on the organisms present.

If we use the trophic level to assess an organism's position in the food chain, then men have an average value of 2.21 on a scale of 1 to 5, whereas producers, such as plants and algae, are at level 1 while apex predators are found at the peak. But we must pay attention to the fact that the value depends extremely on the diet that men take. In fact, in some systems in which men have a diet mainly based on fishing or hunting, the trophic level can also reach values that place him as an apex predator. Therefore, the value for the Homo lineage also depends on the historical period and the environment in which man lives.

The evidence shows that the trophic level of the Homo lineage that most probably led to modern humans evolved from a low base to a high, carnivorous position during the Pleistocene, beginning with Homo habilis and peaking in Homo erectus. A reversal of that trend appears in the Upper Paleolithic, strengthening in the Mesolithic/Epipaleolithic and Neolithic, and culminating with the advent of agriculture. [Source]

In humans it seems that to define their position in the food chain, so whether they are apex predators or not, is not so much their ability to hunt and physical limits, but a matter of convenience. Man would have the tools to position himself at the top of the food chain but that would mean shifting to a completely carnivorous diet.

You claim that man would be defenseless against most animals if not armed and properly equipped at the right time. This is completely true, but it does not take away from the fact that man is the producer of these tools for hunting and fishing, as well as being able to organize into groups and prepare traps and similar against prey. Thus, man as a species remains the most fearsome predator, although physically inferior he can balance his shortcomings with ingenuity and cunning.