There are several logical errors in your initial idea.
"I am hearing that it is suicide to release your game without any marketing and that’s fine, that makes perfect sense. However, I am also hearing people who have released games saying that their sales on Steam were much much better than Itch.io. This doesn’t make much sense to me and it is a contradiction in terms of the necessity of Indie Game Marketing."
No, this is purely a statement about the quantity of people that use each marketplace. If I do no marketing, sell 50 copies on Steam, and 1 copy on Itch.io, it is true that my Steam sales were much much better, but it says nothing about how valuable marketing would have been, if I'd done it. I could have sold 500, 5000, or 50,000 copies on one or either platform with marketing. It's a completely separate issue.
"If my game will only go as far as my marketing efforts"
Again, it's a mistake to try and draw a direct logical comparison between marketing and success. A few games do well without marketing. A few games almost entirely rely on marketing. But most games that do well have at least some marketing, and a ton of the failed games are from developers who put their game up on Steam, then said "so, how do I market my game with zero budget?" Marketing correlates with sales, because it increases your audience and your sales can only ever be a subset of your audience. It's not a guarantee, but it's not worthless either.
"If Steam does, in fact, boost sales simply because it is Steam, then that implies that marketing is not 100% important."
To reiterate, the fact that one factor (in this case, 'being on Steam') increases sales does not necessarily mean that (a) it always increases sales sufficiently to be worthwhile, or (b) it increases sales so much that extra marketing wouldn't help.
"If I am bringing the customers to my game's storefront....if I am the only one putting all the marketing effort and marketing work into selling my game, why should I give 30% away to Steam?"
Because it's not a binary situation of one extreme or the other. Sure, your marketing might drive some people to Itch and you get the full benefits. Some people however, will only buy their games from Steam, so you might have them as an interested customer but never make a sale. Wouldn't you have preferred 70% of their money than none at all?
"If releasing a game on Steam, with no marketing at all, will result in $0 sales, why bother with Steam at all?"
Again, it's not an extreme, or a yes/no situation. Being on Steam means several extra opportunities to access customers. Some of those customers would use Itch.io. Some would not. Sometimes being on Steam means you get on the front page and reach thousands more people. Sometimes it doesn't.
"So which is true here? Releasing a game with no market is guaranteed to bring in $0 sales or does releasing a game on Steam mean that some sales will come in with no marketing effort at all, simply because you are on Steam?"
Obviously I'm repeating myself but neither is true. It's not black and white. These things all interact. There reason you can read almost contradictory reports because everything is on a sliding scale and because it differs from one game to the next. All you can do is maximise your chances, and that means doing whatever marketing you can, and considering whether the 30% cost of Steam is worth it.
(P.S. Please don't launch at $4.99 unless your game is shovelware. Price higher to begin with, reduce in the sales later.)