And an OEM unused battery from 2008 isn't necessarily better than a new random 3rd party one. Batteries degrade over time especially if not stored correctly and one sitting around in a closet likely wasn't.
An OEM unused battery (i.e., one still sealed and, for sake of this discussion, stored properly) is going to, from the factory, still retain a sufficient charge to keep the cells/packs from being discharged entirely — a situation which isn’t good for lithium-ion.
To wit, I bought an OEM battery for an A1181 MacBook in 2021 which was probably manufactured in or around 2011,
based on the serial. When I dropped it into the MacBook, there was still an ample charge remaining — something around half-charged, as memory serves. This isn’t bad for a decade-old, NOS, OEM battery. I’ve had similar, positive outcomes with batteries for iBook G4s (which, likewise, use lithium-ion batteries).
Although it is
possible a random third-party new battery may be in better shape, when new, than a poorly stored, still-sealed, OEM battery, it doesn’t obviate the core point that most third-party vendors of batteries for which the OEM part is no longer sold by Apple aren’t going to be up to the standard. I can attest this from many, many applied experiences across multiple Macs — from clamshell iBook G3s to aluminium MacBook Pros to unibody MBPs, and several models in between.
In ye olden days of whenever, these half-baked, third-party, fly-by-night batteries (from companies like “Upgenspof1086453209” on aliexpress) would be designated, bluntly, as “counterfeit” — not even suitable to be classed as a “third-party on par with first-party”. Their performances, over even limited time periods (like, in as little as two months), tend to fail the basic standard (and, frankly, consumer expectation) of an OEM-calibre battery.
And this is why, when shopping for third-party batteries you expect to last for longer than a couple of months, it’s incumbent to do your homework and to look for the suppliers which have a lengthy reputation for delivering decent batteries with a sufficient warranty to go along with it.