The "s" which you marked in line 5th of the marked paragraph should be 5f, instead.
As we already know, that I.E. is directly proportional to Zeff., if the effective nuclear charge (Zeff.)is more, more energy will be required to pull out that electron from the orbit.
According to slater rule, if we calculate the value of Zeff for any of the early lanthanoid and actinoid, separately, we find that Zeff. for actinoid is less than Zeff. for lanthanoid. That means that 5f electrons have a greater value of shielding constant as compared to 4f electrons. Thus, 5f electrons are available for bonding in the actinoids.