Re: Did Tobi really attack Konoha 16 years ago?
Cue someone with an Obito joke.
Minato has no reason to know that it was Madara behind the mask or know him as Tobi. That's why he wasn't able to give Naruto more.
Minato has no reason to lie to Naruto about the attack; Madara had every reason to lie to Sasuke. One of the factors leading into the decision to massacre the Uchiha was the suspicion that an Uchiha was behind Kyuubi attack, taken with the evidence known by the elders that the Sharingan can control the Kyuubi. For Madara to reveal his involvement would turn Sasuke against him.
That's it. The rest is the old debate of "is Tobi really Madara" which has been discussed a lot. My nutshell - until anyone proves otherwise, I'm going with it. Simplest explanation rather than having to explain why he would pretend.