ORIGINAL: Rodney
The main reason for the added strength of the slower epoxys is NOT that fact it soaks in further but because the molecular chains formed as it cures are longer in the slower curing mixes. This is what increases the strength. Of course, if you tried to put 5 minute epoxy that is already starting to cure in a joint, it will be weaker because of how well it binds with the material being joined so (it soaks in better) is partially true but not the full story.
That is the reason for the epoxy being stronger, not the joint.
If any glue soaks twice as far into the wood and then hardens like epoxy, you've really got an excellent joint. Because very often the exterior area surrounding the epoxy is a function of how far it penetrates. And if you can double the area being held (by the epoxy in this case) you've more than doubled the strengthened area around the joint. The epoxy in the immediate joint area is going to be massively stronger than the wood whether it's 5minute or 30minute. What you're gaining from the extra penetration is a significant area of strengthening of the wood that supports the joint or the joint supports.
Truth is, epoxy isn't the best choice for the majority of joints in our models. However, the places it's good for are areas that benefit from area strengthening, like firewalls. and that brings another point.........