He had to explain it. A lot. As this was quite the scandal at the time.
He himself reasoned and justified the deal, for example in his memoirs.
Franz-Josef Strauß: "Die Erinnerungen", Berlin, 1989 (p 470–496).
He reasoned that existential hardships might lead to uprisings in the East which the West could not support, thereby leading to their demise and raising the threat potential for warlike tensions.
He also thought that to be a bargain, both within the West and the East. The concrete negotiated details like easier border regime, facilitated travel for a subset of the population or agreements on armament, economic cooperation or environmental issues –– were much less important, according to him, than his manoeuvring against the Socialists in both states and the possibility to gain manoeuvrability against the even farther right than him in the West!
The Party newspaper Bayernkurier also stated in its issue from 16. July 1983 an account of official goals. It was still stated to aim at unification in the long term, while getting pragmatic solutions to concrete problems now. And from Western perspective that meant particular "humanitarian quid-pro-quos".
And in a variation of the old Vulcan proverb that 'only Nixon could go to China',
Ein Glück für Strauß, daß ihn kein Kritiker namens Strauß verfolgt!
–– Roswin Finkenzeller, Am liebsten wäre er immer unterwegs, in: FAZ, 29.7.1983.
His own memoirs are remarkable as a source as he died before the state merger took place and thus he did not have the chance to rewrite his own history with that hindsight. How much rewriting took place regarding this deal can be seen when reading the contemporary accounts around that date in old newspaper analyses, like DDR-Milliardenkredit: „Das ist ja ein Ding“ Die deutsch-deutschen Kontakte des Franz Josef Strauß, Spiegel, 1983 or an interview with Strauß and compare them to the mythical narrative now established.
In contrast to the quote from the question:
This move, in violation of longtime CSU/CDU policy to allow the East German economy to collapse naturally…
He declared in the interview just linked:
I didn't stain my hands because there's nothing to stain here. But anyone who opposes this policy must stick to the truth. He must then say: I am against the German policy of the Federal Chancellor and his Federal Government. That is the only thing he can say and then attack the Federal Government. If I agree with this policy on Germany, then we cannot say that I pushed it through against the will of the Federal Minister for Economic Affairs. Anyone who is against me is also against Kohl.
There certainly is some truth in there. How much is an exercise for the reader.
