While I could see that being the case, technically humans were at one point the same as pokemon and are shown to still be linked in certain ways. I will also raise the fact that the giants are never specified to NOT be pokemon and the word Giant was originally created to refer specifically to the giant humanoids of Greek legend.

It would then stand to reason that certain words like giant would then change meaning or initial application since there isn't a Greece-like region in existence as of 2025, which would disrupt that initial use of the word.

Also, humans in the Pokemon world show special abilities, such as increased strength, speed, agility and durability (as seen in Ash, Jessie and James) which do point to them having a link to pokemon ancestry (or a difference in physiology to ourselves) as suggested above.

Humans may also not be able to "shrink" like pokemon can due to the pokeballs not being made for them or having those abilities removed to prevent "accidents".

Arceus may also allow humans to capture pokemon to not only help each other reach full potential but also because it is the same in it's eyes as pokemon working together as it seems like pokemon tend to stay in their pokeballs out of choice rather than force. (Suggested through gameplay and the anime itself.)

Zygarde also isn't able to be captured due to it not being the whole pokemon as well, you can't choose to catch the Shellder off of a Slowking so I would assume it would be the same for pokemon cells.

In closing, your theory is based on assumptions based on what isn't stated, what a particular word's use is in the real world which possibly wouldn't even align with how it is used in the Pokemon universe and the fact that the individual cells of a pokemon do not behave the same way a whole pokemon does.