The Atari 2600 has two 8-bit wide player sprites, and it appears to be these sprites which are used to draw these aliens.

There are a few different ways to produce many copies of one sprite; controlled by nusiz0 and nusiz1 . If you don't want the sprite stretched, then there's a choice between:

one copy of the sprite

two copies of the sprite, close together, far apart, or medium.

three copies of the sprite, close together, or medium

I suppose it's possible they used player 1 and player 2, both together, three copies of each, to get the six aliens on a row. But what about when an alien gets shot down? Some kind of routine needs to work out how to configure each sprite, with number of copies and distance between changing every time. Has that ever been disassembled? I'd be interested to figure it out.