I remember watching a documentary on that. Those scientists faces were pretty shocked looking when they fired that piece of foam and it tore through the tiles like a rock through a window. Amazing they had never considered that a possibility.
They have - NASA engineers even wrote a software to estimate the impacts on the Shuttle. There had been really bad previous debris strikes in the past. Especially when a part of the SRB nose cone hit the Space Shuttle Atlantis during STS-27.
(See the white damage to the black tiles?)
NASA managers had been badly shocked by that.
What they had to test is: How bad would the damage really be on RCC tiles, because the software did not calculate those. It was always assumed that an impact there could only cause minor damage on RCC.
The tests showed, that RCC can be badly damaged, if bad luck strikes. Most impact angles resulted in the expected behavior, the RCC remained intact or was only damaged within tolerance. But in some few angles and impact velocities, the RCC tiles had holes in them.
Still, the engineers who knew about the properties of RCC had been far less surprised than the managers.