You're right that mapping it as a normal highway=path is misleading for routing purposes.

The La Engaña tunnel is an abandoned railway tunnel, completely unlit, partially flooded, and with a major internal rockfall roughly halfway through the tunnel that makes passage very difficult. Traversing the tunnel is dangerous and strongly discouraged.

The south entrance (Burgos side) is currently accessible, but the tunnel cannot realistically be considered a normal pedestrian or cycling connection.

The surroundings of the north entrance (Cantabria side) are currently being adapted as a greenway / cycle route, which may also explain some of the recent path mapping nearby.

I'll update the tagging to better reflect the real conditions inside the tunnel and reduce inappropriate routing through it.