actually not true
Why Manny Pacquiao packs world's most powerful punch: science behind boxer's right hook
Pacquiao because of his speed punches DAMN hard at approximately 806 ft pounds. Tyson did punch harder yes, due to the same physics. While never formally tested analysis of fight videos showed a peak of 1,178 lbs. Rock Marciano, as comparison for both, was formally tested at 925 lbs.
Now you might say that "well Mike punches are 46% more powerful!" Yes but lets look at what 806 lbs for force can break...
Throat- 300 lbs of force
Zygomatic-800 lbs
mandible - 800 lbs
maxilla - 500 lbs
Lat. Maxilla - 700 lbs
"nasal bone"- 200 lbs
Cervical vertebra - 500 lbs
area above the ear - 650 lbs
So while Speed is only part of the equation it is an important part, and yes Pacquiao could hurt Lesnar, just not as much as Tyson. Also as Tyson typically fought at around 220lbs and in the test we see Manny weighing in at 152 lbs. we see the importance of speed to the equation.