In general, the argument holds some water, but it ignores the fact that human bones can take a lot more than their own weight today — depending, obviously, on how they are stressed (they can probably take maximum force along their lengthwise axis, but depends on the bone too — femur is said to take 30x the body weight, and we've got 2). Also, their structures — just like bending an edge on thin metal sheet — can significantly increase their carrying strength without changing the weight.

I'd also note that it does not take a lot for someone to be a "giant". When average height was 1.6m, someone 2.2m tall was certainly a "giant" (they are even today, but it means they'd frequently be asked if they are playing basketball :)).

But even if they were 4m tall, they'd certainly be called a giant and certainly regular human bones could handle weight up to say 450 kg. Heck, someone brought up horses and cows, and horses certainly jump off two hind legs and land on two front ones, putting a lot more than 600kg of their standing weight on them.

So in short, Scientific American is right to bring up the fact that someone like Galileo would put his genius to folk topics back in the day, we do have a bit more to lean on today to come up with even stronger conclusions — so they'd better highlight how his thinking compares to what was known back in the day.