We had 3 "basement waterproofers" over to check the basement (due to a history of small leaks during heavy rains and once when the hose leaked and we found a puddle of water at the edge of the perimter basement wall). During the last heavy rain we noticed a small trench full of water about 2 feet from the wall we had a small leak under our deck. Right after the storm the water seemed to seep into the ground and then right on cue, was ni our basement along the same wall. All waterproofers had the same opinion, surface water pooling outside the exterior basement wall getting pulled through the concrete block into the home.
After further investigation we realized our gutter downspouts were completely clogged! We have had 8 inches of rain and the gutter person said that means thousands of gallons of rain had been pooling at our basement wall where we noticed all the water pooling as they gutters were overflowing and that this definitely could be our whole problem. Could it really be this simple?? Should we still regrade under our deck or just wait and see what happens now that the downspouts and gutters are clear? Has anyone had a similar experience with gutter being clogged leading to a significant amount of water leaking in and then it was resolved by ensuring clear gutters and downspouts?

Yes, it could be that simple: cleaning downspouts and gutters is step 1 in avoiding basement leaks. Water has to be carried away from the house.

I agree with rtw. I t can be that simple. Grading is as much or more important.

Keeping the gutters clean can be a huge job depending on the number and types of trees closeby.
I had about 100 trees near my last home and wasn't willing to pay for gutter guards (which do work) but cost more than 4 10 per foot. I tried all the cheaper versions from home improvement places and none of those work for serious conditions.
I finally built my own from racecar metal and pvc pipe and it looked and worked beautifully.
It's all a matter of surface tension causing the water to go in the gutter and the debris to fall to the ground. It works. In about ten years I never had to remove the cover and clean anything out.
The leaf raking, leaves to be blown off the roof, dampness, sticks, and sap finally chased us away as we got older.

Water traveling underground can also cause problems also, but it can be controlled.

That should have said $ 10 per foot, and that was based on a long ago estimate I got from the leading company that does it.