There is several anime whose opening/ending are
made of cuts that are from the anime itself. It 
isn't especialy unusual.

Why a cel coming from such an opening shouldn't be 
concidered as one ?!?

OP cels have more values for 2 reasons only :

1) Because they are shown in (nearly) all episodes
   It's the same for eyecatch for example.
   (rather pointless for a movie or single OVA)

2) Because collectors give an extra value to an
   arbitrary small part of the anime. In the same
   way that a specific scene may be considered
   more value. Or because the character wear
   some differant clothes than from the rest of
   the anime...

Based on this, a cel from an opening composed of 
cuts from the anime is not differant from one 
coming from an "original" opening.

(And no, quality is not a criteria for OP cels : 
Some OP are of higher quality than the anime. 
Other are of lower quality. Most are of the same 
general quality)

Cheers,
Yann Stettler