Learn Spanish (Latin America): De hecho, el examen fue fácil.

Not quite. While the current lesson focuses on es decir and o sea (which mean "that is to say" or "in other words" to clarify something), de hecho means "in fact" or "as a matter of fact". You use it to add a strong confirming detail or a surprising truth, rather than just rephrasing what you already said.

Why do we use fue (preterite) instead of era (imperfect) to describe the exam as easy?

We often use the imperfect (era) for descriptions in the past, but an exam is a specific event with a clear start and end. When you evaluate a completed event as a whole (like how a test went), you use the preterite (fue). If you were just setting the scene in a longer story, you might use era, but as a final summary of the event, fue is exactly right.

Since hecho comes from hacer (to do/make), how does it mean "fact" here?

Hecho is indeed the past participle of hacer (done/made), but it also functions as a noun meaning "fact" or "event". So the phrase de hecho literally translates to "of fact", which is exactly how we get "in fact".

Why does fácil have an accent mark on the a?