Role of "that" in the beginning of a sentence?

IN EARLY 2020, with the coronavirus pandemic tearing across the world, most people thought it unlikely that a vaccine would arrive any time soon. And as work to develop vaccines began, there were dire warnings of the difficulties ahead. That is why it is so remarkable that, heading into 2021, it seems likely that one or more working vaccines will soon be available. That this can be said with such certainty reflects the number and diversity of approaches being taken.

What is the role of "That" in the beginning of the last sentence? Under what circumstances a sentence can start with "That"?

Source: https://www.economist.com/the-world-ahead/2020/11/16/the-covid-19-vaccination-programme-will-be-the-biggest-in-history

Solution 1:

The role here is to be a shortened version of "the fact/idea that...".

The fact that this can be said with such certainty reflects the number and diversity of approaches being taken.

Another example:

The idea that my drawing of a cat can be mistaken as a bird is proposterous. You must be joking.

Which can also be said as:

That my drawing of a cat can be mistaken as a bird is proposterous. You must be joking.

As for when to use "that" this way, Merriam-Webster describes it like so:

Used as a function word to introduce a subordinate clause that is joined as complement to a noun or adjective.

For additional insight: 'The fact that' versus just 'That'