Being alone means you are established firmly in the here and the now and you become aware of what is happening in the present moment.
You use your mindfulness to become aware of every feeling, every perception you have. You’re aware of what’s happening around you in the sangha, but you’re always with yourself, you don’t lose yourself.
That’s the Buddha’s definition of the ideal practice of solitude: not to be caught in the past or carried away by the future, but always to be here, body and mind united, aware of what is happening in the present moment.
That is real solitude.
Thích Nhất Hạnh