It's funny that this phrase has had such staying strength, while the key word in it, "powerhouse," has fallen out of fashion. The more modern American English way to phrase it would be "the power plant of the cell."
So maybe the original usage has been subsumed by "power plant", but I think the word has alternative meanings which persist.
Powerhouse is a common way to describe an athlete, a high performance engine, or a very strong stock buy -- it has just moved away from the infrastructural uses.
These things do happen. I was in my 30s before I learned what the "firewall" of a car was...