～そばから and its する form


Why does [this site](http://nihongonosensei.net/?p=3730) say that そばから "Can be
used only with past tense" then use it with ている？

[![enter image description
here](https://i.stack.imgur.com/JXzBw.png)](https://i.stack.imgur.com/JXzBw.png)

The usage note:

> 後件は良くないことが多く、 **過去のことにしか使えません** 。

The example sentence:

> 雪かきをするそばから雪が積もり始めている。




It doesn't say "past tense" but 過去のこと and it seems to mean "what has already
happened", here. It's certainly misleading, though.




It doesn't say "past tense" but 過去のこと and it seems to mean "what has already
happened", here. It's certainly misleading, though.

