"Visita alguno de ellos."
Translation:Visit one of them.
That's a bit restrictive. You can use "alguno de ellos" to mean "some of them" "any of them" or "one of them." Perhaps, SpanishDict (I couldn't find the reference) was illustrating what they consider better usage to minimize ambiguity, since "some of them" isn't the same as "one of them."
This is one dictionary entry, specific to the singular form, from RAE:
Expresa un número escaso e inespecífico de personas o cosas. U. referido a un sintagma nominal mencionado o sobrentendido, o para aludir a un sintagma pospuesto introducido por la preposición de.
Expresses a limited or non-specific number of people, animals or things. Used to refer to a noun phrase mentioned or understood, or to refer to a subordinate phrase introduced by the preposition de
I think that is is because both "alguno" and "ellos" don't have to be people. Within this context, this sentence could be talking about places, in which case you wouldn't use the personal "a". Visitar also isn't one of the verbs that always has to halve "a" afterwards, like "ir".
It could also be that they're people, but since the identity isn't important, this doesn't get a personal "a." I'm told that in sentences like "call a plumber" sometimes that plumber basically doesn't count as a person, because you don't care at all about his particular identity beyond "plumber."
Not a specific rule but very generally (depending on context), uno most often refers to an object and alguno refers (most often) to a person. Not to confuse the issue but they can be synonymous but they are not synonyms. It might help to think of "alguno de ellos" as a phrase meaning "one of them (people)".