I think you're quite right that most content gets likes and engagement from people promoting their company, their mates in the industry and people whose attention they want to attract, and usually doesn't spread much beyond that. That's the case whether it's genuinely interesting or generic promotions.

But the "viral" content seems to be something else entirely: as you point out a lot of the people are rubes running pre-product start-ups or consulting, and surely there are more people wanting to impress people with actual budgets and teams and products. Feels like they're successfully catering to an algorithm calibrated for bored but easily impressed scrollers (as well as other rubes and bot-operated accounts that want to share their equally unlikely takes on B2B sales) rather than their network.

Recently I saw a recruiter posting side by side screenshots of the engagement with a high effort collection of industry info she'd compiled with infographics and links, and a copy/paste of an unfunny meme with a tagline applying it to her industry. You'll never guess which one had 10x the engagement...