When you paraphrase you hope to give a sentence with similar meaning. There are very few pairs of sentences with exactly the same meaning, register, tone and nuance.
"Must have" is used to indicate deductions or conclusions from evidence.
The pavement is wet, it must have been raining
It implies that you did not directly observe the rain, and so suggests slight doubt:
It must have been raining.
No, I think the kids have been playing with water.
There is a similar effect from adding "I'm sure..." or "... definitely ...". The fact you feel the need to say "I'm sure" actually makes it sound less certain. Compare:
Peter is a good boy. (simple assertion)
I'm sure that Peter is a good boy. (Why do you need to say this, unless there was some doubt).
Saying "The Smiths must have bought this house" means that you have worked it out from evidence and reasoning. It is close in meaning to "I'm sure that the Smiths have bought this house." I'd accept it as a reasonable paraphrase, though not exactly the same.