Hello, I have never worked in a sales department and have only ever worked around people who tell me second-hand information about P&L metrics for the games industry. Allow me to give you all my take on this:
I think, beyond the initial translation probably lacking a bit of context or clarity in some sense, that the interpretation from a lot of people here is too literal or rigid. 1 million units by most accounts is not a "huge hit", but 1 million on a low budget does not mean a good profit or a minor success. Sometimes you can be a small hit, a cult classic. Sometimes you're a pretty big hit, you sell 10 million. On rare occasion, you're a gigahit and you sell 30 million! And so on...
IMO I think the reason Miyamoto used that 30 million metric specifically is because those titles (Smash, Kart, BOTW and AC) have been long-term sellers and have greatly aided the annual and quartely revenues of the company, meaning their cash flow can generally sustain operations in parallel, as opposed to having large, inconsistent increases in revenue relative to game launches. Nintendo is a huge company, it takes a lot of money to keep the lights on there.
Keep in mind, Miyamoto is talking to Shigesato Itoi. This is not a room of shareholders or business people, and Nintendo's sold very few 30M-sellers. I don't think they LITERALLY have to sell 30 million, he was likely using such a large number to give an example of what that sort of success looks like to an audience of people who's heads are casually removed from the biz.
In regards to this, TOTK's case is nuanced, and it's that nuance which is kind of why I think it's frustrating that people will jump to conclusions off of something as negligible as a loose, generalized statement made to a casual audience of viewers from someone who doesn't manage sales. TOTK had a HUGE burst of sales initially (18 million!) and immediately trailed off, having grown very little in the year since it's release, whereas BOTW saw consistent growth following its launch. In some sense, that's pretty disappointing. On another though, supposing Nintendo spent 70 million dollars on it, they've made a rough profit of nearly 1 Billion~ off that investment. That's still a HUGE success.
There can be endless discussion on why people think TOTK underperformed those long term sales, but fact of the matter is that TOTK doesn't have legs, and Nintendo can find that disappointing while ALSO considering it a financial success. Things have nuance.