It's much more than gui web apps, it also includes native mobile apps, headless services, integrations, etc.
And from a business value they're very different: COTS you do not want to modify/adapt to your busines, otherwise you get problems upgrading, etc. Low-code is often used to innovate and differentiate, as a programming layer to quickly change COTS to your business needs.
Disclaimer: working for a low-code vendor.