I am no expert on Fuller, and I suspect he is weak in a number of areas. But he may be making a distinction between capacity and ability somewhat like Pink does below:

"When we speak of the sinners inability, we do not mean that if he desired to come to Christ he lacks the necessary power (or capacity) to do so. No; the fact is that the sinner's inability or absence of power is itself due to a lack of willingness to come to Christ, and this lack of willingness is the fruit of a depraved heart." (A. W. Pink, The Sovereignty of God, Baker, p. 187)

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Jim
1689 LBCF
Independent Bible Church
North Texas, USA