> I was wondering if it is possible for a startup to be too lean?
I'd say that's very dependant on each individual scenario. For instance, if the MVP(minimal viable product) is built by just the technical founders in spare time and there is no time limit/urgency, then the answer is that it can't be too lean. On the other hand, let's say that the MVP is still built by the founders, but they are working on it full time, then the startup can be too lean. The less developers working on the project, the longer it'll take to finish the product. Even if the founders are okay with not paying themselves, their personal bills still have to be paid, so there's a built in deadline that has to be met there for positive cash flow/ investment.
The same would apply to if there were paid hires. You can pay one employee a salary of X and finish the project in Y amount of time, or you can hire 2 employees for a total salary of 2X and finish the project in Y/2 amount of time. If salary is your only expense then it doesn't matter if you're lean. However, if you need to pay rent, electricity, hosting, etc, then the faster the project is done the better and being too lean may hurt you.
Again, all depends on the case at hand. Is there cash flow, are employees getting paid, is there an urgent delivery date, are there external expenses, etc.