It seems to me that τοῦτο ("this") is the subject (neut. nom.) and αὐτοὺς ("they [themselves]") is the direct object (masc. acc.).
Here's the A.V.:
For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water:
λανθάνει is conjugated in the 3rd person, present tense, active voice, indicative mood, singular number. In the A.V.'s translation, it appears that the subject of the verb is "they," which would be the plural αὐτοὺς. This doesn't seem right to me. How about anyone else?
θέλοντας is masc. acc. plural, so it appears to modify αὐτοὺς which is fine.
The same phenomenon (#1) appears to happen in 2 Peter 3:8:
Ἓν δὲ τοῦτο μὴ λανθανέτω ὑμᾶς ἀγαπητοί
which the A.V. translates as,
λανθανέτω is 3rd person sing. imperative. But, the A.V. translates it as though the imperative is 2nd person plur.But, beloved, be not ignorant of this one thing