Whether "所" means "actually"? Is that meaning really used in Modern Chinese?
My answer to your question is No. The word "所" does not mean "actually" in any sense.
The word "所" has the following meanings,
(1) A noun = a place. For example, 厕所， 诊所。
(2) A pronoun = that (or which) in a clause. For example,
他所说的并不真实。= What he said is not true.
(3) A measuring word (or a classifier) = a number of places. For example,
县城有两所高中。= There are two high schools in town. (no need to translate 所 into English).
(4) Used in a passive sentence as 被--所--. For example, 他被生活所困。困。= He is confined by making living.
I found an example that translates the word "所" into "actually". But I think it is a misunderstanding of the usage of the word "所". I would like to give you a scenario to explain. For example,
(1) 虽然这本字典有一万个词汇，但是实际上学生所需要学习的只有三千个。
(2) 虽然这本字典有一万个词汇，但是学生所需要学习的只有三千个。
(1) and (2) have the same meaning = Even though there are ten thousand words in this dictionary, the vocabulary that a student needs to learn is actually only three thousands.
As you can see that the meaning of actually is indicated by 实际上. In (2), the word 实际上 is omitted, but it still has the same meaning.
The meaning of actually is not from the word 所.