Should we teach by "subject" or by "topic"?

Continuing with studying outside sources on education, I came across a short little article on Finland's practice of teaching by "topic" rather than "subject".

Unfortunately, the article was not very articulate on what they specifically meant by "topic". Their basic argument was that instead of teaching "a history subject in the morning and a geography class in the afternoon," their students would instead learn through application of a specific "topic". The article's example of a "topic" was what they called "cafeteria services". This course would be in place of a common vocational course, and would include lessons in "elements of math, languages (to help serve foreign customers), writing, and communication skills."

While I agree somewhat with the idea of not teaching strictly by "subject", the idea of teaching by "topic" doesn't seem to me (with my VERY limited experience, I admit) to be much of a change. Yes, it furthers the individualization of education, something which I readily support, but it seems to me that it just "re-defines" the term "subject". Instead of "subjects" being "science, math, engineering, and math," a student will now define them as "carpentry, politics, and kitchen service". The difference is minimal.

Yes, you get to hear me praise Montessori again.

Yes, I know I'm a biased preacher due to my relative inexperience with many things outside the Montessori system. 

What I would like to add about said system is it's lack of what I would call a "period-schedule format". This allows them to keep some of the "subject" system intact, but allows the different subjects to melt together into a single atmosphere, which I believe encourages these children to more creatively combine and use these skills.

I'm still kind of uncertain about replacing "subjects" with "topics" as I still don't see the difference. But if any new information can be found to support this system, I would be more than happy to research it further.