Given that the soviet union fought in afghanistan, why was it considered a proxy war?

It was a "proxy war" because it was a conflict in which the United States did not directly engage in battle.  

The USA and the USSR were on opposing sides, but it was  not a direct war between the USA and the USSR.  It was a "proxy war" within the larger Cold War situation.

Explanation:

The "Cold War" had that name because the two superpowers (the USA and USSR) did not fight a direct "hot" war against one another.  They engaged in a protracted stand-off against each other, and had an arms race of nuclear weapons and military strength.  They also supported "proxy wars" where they took opposing sides in conflicts happening in other parts of the world, such as the Six Day War in the Middle East in 1967, or the Soviet-Afghan war in the 1980s.

In the Soviet-Afghan War (1979-1989), GlobalSecurity has reported:  The United States supported the Afghans fighting for their country's freedom in the 1980s - as did other countries, including Saudi Arabia, Pakistan, China, Egypt, and the UK."  So it served as a proxy war in the larger Cold War conflict between the US and its allies over against the Soviet Union's sphere of influence.

it was a conflict in which the United States did not directly engage in battle.

Explanation:

it further strained relations between the north and the south and convinced