I've been wondering the same.
Spent some time looking into it further. The Holiday 2001 issue itself is atypical - only 10% or so is actual GameShark Magazine and what's there is very thin in regards to content. It consists of a brief 1/2 page editorial and 9 1/2 pages of codes. No reviews, interviews, etc, and none of the usual trappings like a product order form and mailing address section. The IGN portion of the 'zine is a separate animal altogether with its own creative staff, numbering, a UPC code, etc. Its only noticeable ties to GameShark - besides the cover caption - is the bundled sampler disc and advertising being almost 100% GameShark related.
To be honest, the GameShark Magazine portion of this issue comes across as a glorified advertisement - like a big corp supplement thrown in. I'm speculating but it looks as if IGN cut a deal with GameShark and their magazine division and this issue is the result. Perhaps IGN created their own portion of the 'zine internally and GameShark Magazine - having the infrastructure already in place - sponsored it and handled the actual publishing.
This would help explain what looks to have been a dual distribution. I know the issue was only sold by IGN directly through their website - both as a premium subscriber incentive and separately - but it looks to have also been freely distributed by GameShark Magazine as well. A number of sources seem to back this up - Wikipedia, the Internet Archive, RetroMags' own gallery, the Holiday 2001 editorial, and a Nintendo Age post. And going a step further, perhaps this issue was actually distributed to GameShark Magazine's mailing list as the last and final issue sometime following their March 2002 issue? Maybe they had some overstock or whatever and thought this would appease their subscriber base? Hopefully, a former GameShark Magazine subscriber is reading this and can clarify the details.
The issue definitely is a flip book, btw. (Sorry for that.) The cbr doesn't really reflect this as both sections are oriented right-side up. Not exactly an accurate preservation but now we don't have to flip our tablets over while reading.
If it turns out this issue was never distributed by GameShark Magazine, I'd be hard pressed to categorize it as such. As mentioned before, it feels more like a supplement than full-fledged issue. It'd be better categorized by its IGN roots - with a GameShark Magazine reference in the publication dbase & issue index for clarity - unless RetroMags allows for double entries to accommodate flip issues.
If it was distributed by GameShark Magazine, it should probably be considered a special issue outside the usual publication schedule.
I made some updates to the OP with Holiday 2001 details and also added GameShark Magazine to the magazine dbase. We'll need a download section and a publication dbase entry eventually.