This is correct. A demon can never enter the spirit-man...it is sealed. However, he will enter the inner-man (soul) and the flesh (mind) of a Believer from a wounded soul and trauma. Sin gives access to the demonic. These are called strongholds, bondage, and yokes.
A Spirit of Infirmity
10 Now He was teaching in one of the synagogues on the Sabbath. 11 And behold, there was a woman who had a spirit of infirmity eighteen years, and was bent over and could in no way raise herself up. 12 But when Jesus saw her, He called her to Him and said to her, “Woman, you are loosed from your infirmity.” 13 And He laid His hands on her, and immediately she was made straight, and glorified God.
14 But the ruler of the synagogue answered with indignation, because Jesus had healed on the Sabbath; and he said to the crowd, “There are six days on which men ought to work; therefore come and be healed on them, and not on the Sabbath day.”
15 The Lord then answered him and said, “Hypocrite!b]'>[b] Does not each one of you on the Sabbath loose his ox or donkey from the stall, and lead it away to water it? 16 So ought not this woman, being a daughter of Abraham, whom Satan has bound—think of it—for eighteen years, be loosed from this bond on the Sabbath?” 17 And when He said these things, all His adversaries were put to shame; and all the multitude rejoiced for all the glorious things that were done by Him.
Jesus said this woman was a daughter of Abraham, which means she was saved. If you look up the word infirmity in the Strong's, it means soul. This woman soul was wounded from sin.
According to 1John 4:4 a demon can not enter the soul or mind as greater is He that is in us then he that is in the world. The woman with the spirit of infirmity had not known who Jesus was until He called her to come to him and then He spoke to the spirit of the infirmity casting it out of her and after Jesus laid hands on her she understood who Jesus was/is.