On that page, under Country specific / United States, border_type=state corresponds to admin_level-4.

14 of the states had border_type tags, 36 did not.  Every county, parish, and borough in the US have a border_type tag, and in the forum we've discussed that every boundary=administrative should have a border_type tag.

This also allows easy filtering in Overpass.

For example, this query returns over 70 results:

```
[out:json];
area[wikidata=Q30]->.parent;
relation[admin_level=4](area.parent);
out tags;        
 ```

Included are the US territories, the District of Columbia, three Canadian provinces where the US and Canada have a border dispute, several farm management areas, etc.

This query slightly improves the situation:

```
[out:json];
area[wikidata=Q30]->.parent;
// Add boundary=administrative
relation[boundary=administrative][admin_level=4](area.parent);
out tags;
```

But it still returns 59 results, including the US territories, the three Canadian provinces, and DC.

However this query:
```
[out:json];
area[wikidata=Q30]->.parent;
// Add border_type=state
relation[boundary=administrative][border_type=state][admin_level=4](area.parent);
out tags;
```

Returns the 50 states.

All border_type=state are admin_level=4, but not all admin_level=4 are border_type=state.