I saw it immediately as well. Some tells for me are:

- Off-white or sepia toned backgrounds, similar subdued color palette for icons, grey ALL CAPS subheadings

- Serifed headings

- Various "Item: Quantity" lists (Puzzle types: 10, Puzzles solved: 1,951, etc.)

- Middle dot character for separator

One common tell it is lacking is the placement of colored dots or circles in the corners of panels or other UI elements, sometimes animated/pulsing.

To be clear it's not bad, it's a clean and friendly style. It just has that certain look, like a visual "it's not X it's Y".