We know the souls themselves still exist in the Hollow/Arrancar otherwise Ichibe wouldn't have issues purifying people like Grimmjow or Ikomikidomoe, it's directly stated that they contain too many souls to be purified, which is why Ichibe sealed Ikomikidomoe and didn't want to fight Grimmjow.
Even in the actual quote itself, it's stated, google the word "amalgamation".
Their one soul is the result of millions of souls being combined together, again, this is clearly stated both in the manga and secondary canon material.
Aaroniero was unique because he could use all the abilities of all the Hollows he consumed, not because he had multiple souls.
AppleLord truly is the best stonewall the Bleach fandom has to offer.
Lastly, to add onto my first point, it's also stated in the manga that the consumed souls themselves still exist in the Hollow/Arrancar, how else do you figure they have to fight for supremacy against the many consumed souls or are able to lose their mind to one of the other countless souls they contain?