So, there are two ways a weld connection fails.
1) The weld itself fails.
2) The weld is sufficiently strong, but the base material it is connected to is not strong enough and ruptures.
What this paragraph is saying is that it can be difficult to predict #2 precisely for some weld arrangements. Therefore, they take a shortcut and look at the rupture strength of the base material and what thickness it would take for it to match the weld strength. If the actual thickness of the base material is too small then they will say that the base material doesn't work....