I think it's more likely a SIM card slot. Lenovo has included these for quite some time (under the battery in the T460, for instance). That's not something you change every day, so needing a pin to open it makes sense (plus, it prevents against theft somewhat). Also, a lot of modems do not like you removing it while they are powered on, so having the lid prevent its opening sounds like a feature.
The utterly inconvenient access makes me think it must not have been intended for frequent swapping.
IE, it's probably not there to support photographers or drone pilots or any other activity that involves frequently swapping cards, and so it's not quite right to criticise it for being terrible at that. About like complaining that the trackpoint is terrible for CAD.
I have no idea what it's intended function IS though either. I just put a big card in there just because I could, and used it as shared space that both linux and widows could both access. It wasn't necessary nor the best way to do that. That can't be what it was meant for either.
Don't attribute to intent what can be adequately explained with LSD laced vodka enemas. For the last decade the design language of ThinkPads is an atrocious [and successful] process of making their users suffer.
Pretty ridiculous.
It's easier to find the opposite: an external connector for the SIM card, and you leave the SD card in: https://www.aliexpress.com/item/1005003822185179.html
These are often used on phones to use both two SIM card and one SD card on phones that have combo trays... Speak about usability :/