Is ifixit Apple certified for battery replacement

Probably not. Click this link to see the Authorized Service Providers in your area --> https://getsupport.apple.com/?caller=psp&PRKEYS=

If you have an Apple or Best Buy store near you, you'd be safest going there. Apple's battery replacement cost is only $49 and either Best Buy or Apple will use only genuine Apple batteries.

No they are not.

They also do not have Apple parts but sell Chinese knockoffs.

The tools they sell are pretty decent.

No. They’re specifically not because Apple would restrict what repairs they could do. I generally trust them however.

It should be noted iFixit only sells parts or kits, they do not offer repair service.