The problem is that subfolder is a wildly more complex and fragile configuration, so any imagined SEO upside is far more than outweighed by the difficulty of setting it up and maintaining it over time.
If subfolder was as easy as subdomain, I’d have no objections, because ultimately it doesn’t matter.
What matters is whether you produce quality content that people visit and link to.
Everything else is just noise, or people trying to sell you a fake shortcut to success without doing the work of producing great content that people visit and link to. Don’t be fooled!