All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Fatwa 217561, which you have partially quoted in your question, highlights in detail the opinions of the scholars on when it is permissible or impermissible to use hair extensions. Please refer to the Fatwa.
The part of the Fatwa which reads, “Apparently, what is prohibited is extending the hair with human hair for it implies a form of deception …” is in fact referring to the opinion of the scholars who hold that hair extensions (with something other than human hair) are permissible. We have highlighted in the Fatwa the different opinions of scholars regarding hair extensions made of something other than human hair and underlined that it is prohibited for a woman to extend her hair in any way, even if it is with something other than human hair and that this opinion is more prudent. Moreover, it is the opinion held by the majority of scholars.
Evidence that the prohibition of hair extensions is general (i.e. that extending hair with anything is forbidden) and that the wisdom behind the prohibition is not deception is the following report. An Ansaari woman gave her daughter in marriage and the hair of the latter started falling out. The Ansaari woman came to the Prophet, sallallaahu ‘alayhi wa sallam, and mentioned that to him and said, “Her (my daughter's) husband suggested that I should let her wear hair extension.” The Prophet, sallallaahu ‘alayhi wa sallam, said, “No, (do not do that) for Allaah has cursed such women who lengthen their hair.” [Al-Bukhari] The same Hadeeth was reported by Imaam Muslim with a slightly different wording.
Allaah Knows best.