This is based entirely on cumulative emissions, not observed temperature. Although we don't know the exact relationship, I have assumed a base climate sensitivity of 2C per trillion tonnes carbon emitted (Allen et. al, 2009), which implies we have locked in 1.2C already as emissions from 1750 are some 600 billion tonnes carbon (trillionthtonne.org). Of course we haven't actually observed 1.2C yet.
This relationship implies that 750 billion tonnes carbon is 1.5C, which we get to in about 2028, assuming a 600 billion tonne starting point and 11+ billion tonnes carbon per annum.
It may of course transpire that the relationship is not 2C per trillion.