Don't worry it warries from place to place it. May be it is a difftrent list of 33 gods. But yes becouse vedas call indra as aditya so he is.
In the Rigveda, the Âdityas are
the seven celestial deities, sons
- Anśa or Aṃśa
The eighth Âditya ( Mārtanda) was rejected by Aditi, thus leaving only seven sons. In the Yajurveda (Taittirīya Samhita), their number is given as eight,
and the last one is believed to be Vivasvān . Hymn LXXII of the Rig Veda, Book 10, also confirms that there are nine Adityas, the eighth one being Mārtanda, who is later revived as Vivasvān.
8 Eight are the Sons of Adid who from her body sprang to life. With seven she went to meet the Gods she cast Martanda far away.
9 So with her Seven Sons Aditi went forth to meet the earlier age. She brought Martanda thitherward to spring to life and die again.
— Hymn LXXII of the Rig Veda, Book 10
All other puranas in post-vedic mention about 33 gods among whom 12 are adityas and indra is one of them.
In the Vishnu Purana chapter 50, the names of 12 Adityas are given as:
- Vishnu (The head of all the Adityas)
According to the Linga Purana , the Adityas are:
- Indra (The head of Ādityas)
From Shatapatha Brahmana,
Now, there are here thirty-four utterances, called expiations 1 .
Prag âpati, forsooth, is that sacrifice which is performed here, and from
which these creatures have been produced,--and in like manner are
they produced therefrom even to this day.
There are eight Vasus, eleven Rudras, twelve Âdityas; and these
two, Heaven and Earth, are the (thirty-second and) thirty-third. And
there are thirty-three gods, and Prag âpati is the thirty-fourth;--thus
he makes him (the sacrificer, or Yagña) to be Pra g âpati : now that
is, for that is immortal, and what is immortal that is. But what is mortal
that also is Pra g âpati; for Pragâpati is everything: thus he makes him to
be Pragâpati, and hence there are these thirty-four utterances, called
— Satapatha Brahmana 4:5:7:2
Okay this thing is apart from your question but i include it to rigveda doesn't mention of the same type of 33 gods list but something else but like that :—
ये देवा सो दिव्येकादशस्थ पृथिव्यामध्येकादशस्थ ।
अप्सुक्षितो महिनैकादशस्थ ते देवासो यज्ञमिमंजुषध्वम् ॥११॥
O ye eleven gods whose home is heaven, O ye eleven who make earth your
dwelling, Ye who with might, eleven, live in waters,
accept this sacrifice, O gods, with pleasure.
— Rigveda 1.139.11 Translated by Ralph T. H. Griffith.
19 O ye eleven Gods whose home is heaven, O ye eleven who
make earth your dwelling. Ye who with might, eleven, live in
waters, accept this sacrifice, Ye Gods, with pleasure.
Although the list varies from place to place and of course Vedas describe 33 gods but no distinction that 12 adityas,8vasus etc.although in the list of adityas Indra is seen many times in vedas, puranas,Brahmanas etc. so yes Indra is an Aditya.
YDS above the list of adityas from two purana varies at no.8 vishnu purana—parjanya and linga purana—savitra and in whole change one name in both versions as there are 33 gods as per vedas but it does not classify them more and in diffrent-diffrent texts change the list of dities in a sequence like shatpath brahmana where unlike brihadaranyaka upnishad heaven and earth make 32 and 33 respectively so in same way in brihadaranyaka upnishad may have changed the names of adityas.