That guy clearly doesn't know what he's talking about, when he says that defragmenting "can cause harm to your computer." It won't. For the average user, just letting Windows defrag the hard drives weekly, rather than using third party software will be all that is needed. But, neither the Windows defragmenter or third party ones will cause any harm.
Regarding SSDs, it's safe, but also pointless, to defragment them. It's been proven that SSDs have such a long lifespan, that defragging them won't kill them.