Your hypothesis about an advertiser asking wikipedia to alter content surely applies to google search results.
Google indexes other people's content. All Google has to say is, "Sorry, we're not in control of the content others make, our automated systems follow an algorithm we're unable to make one-off tweaks to." It could conceivably cost Google $1MM to make a one-off tweak to their algorithm in terms of programming and testing time.
Wikipedia on the other hand is all content. They have no plausible response other than, "Yeah, it would take 5 minutes to update that but we won't do that for you." Hell, all they'd really have to do is let the advertiser update it as they want and then instruct editors to do nothing.
It really is just different for this and a number of other reasons.
The problem is, if wikipedia did alter articles based on advertisers demands (Which seems pretty far fetched to me), the public would just alter them back. Or see the edits wikipedia is making and put 2 and 2 together.
>and then instruct editors to do nothing.
Yeah good luck with getting wikipedia editors to comply with that request!
A site like wikipedia would likely have thousands upon thousands of advertisers. They wouldn't be dependent on a few big advertisers. If an advertiser came to wikipedia and asked them to change a page, wikipedia would just say "no", publish the details to make the advertiser look like a douche (cue internet witch hunt, boycot naming shaming etc), and not care about the 0.000% temporary drop in revenue.