Good question (OP here). What I mean is that for example, if you build a Chrome Extension (which is the main way my company's product gets used, so I'm familiar with the pitfalls), you are now at the mercy of Google and how they choose to develop their marketplace for extensions, how they choose to enforce user security, and how they choose to change the platform over time. Single vendor platform, similar to the other app stores.
I'm not suggesting you rely on nothing else such as 3rd party CMS or e-commerce features, but I am suggesting that for example if you build a Shopify plug-in, you are at the mercy of how Shopify chooses to develop their ecosystem, and there will be potentially existential crises along the way.
As with all the other rules, it's one you can and should break when it makes sense for your business. There are many thriving startups on top of Shopify's ecosystem, Apple's ecosystem, Google Chrome's ecosystem, and so on and so forth - I'm just urging you to be aware of the hassle you are creating for yourself by choosing such a path, and to balance it wisely against the benefits.