Briefly to begin, I'm going to read the above extremely uncharitably, not in an effort to besmirch aeternum but instead to try and tear down some terrible guidelines about life choices that I've seen going around.
Here follows my translation of the advice above:
> Yes you will probably make more at an established company, but your rate of growth can be much higher at a startup.
You'd personally benefit more from working at an established company, but if you work for a startup you'll be trained on the cheap and then an established company can reap more profits from you.
> I worked at a large tech company for 3 years before joining a startup, and learned more in the first month at the startup than the full 3 years at the large tech company.
The comfort of a large tech company is nothing compared to the stress filled life of working at a startup, having to constantly innovate on your toes and learn your own ways will push you to develop faster - but if you hit a wall you're truly hooped.
> I was able to design and implement a service in a matter of days, whereas at the large tech company those 3 days would easily spent convincing people that the service is needed in the first place.
I saw a need and dedicated some days of labour to filling that need, because it was a valid need I was praised, if it hadn't sold I'd be chewed out for wasting time - and there is no guidance on which needs might be helpful or not... Within a startup you'll be expected to do user surveys and then market your new product even though no time is formally afforded for either.
> Direct exposure to customers also is really interesting and changes you as an engineer.
I assume when you decided to become a software dev you really meant UX designer? If your perfect world is being given a task to do and doing it accurately to a high level of perfection then have fun trying to quickly adapt your highly designed system to literally contradictory client requests coming from the same customer.
> It's definitely a no guardrail environment, in the early days we had a bug that directly cost us a large customer pilot. No better teacher than experience, we did not make that mistake twice.
Losing your shirt trying to make your day job not collapse and then being excluded from any of the benefits when your company goes public is character building.