No it means that at TL6 they are shooting at aircraft with with the same 50cal M2's they use in the ground and naval role just on a high elevation mounting.
The description for Anti-Flyer weapons suggests high rate of fire, ballistic characteristics (+1 Mod to Hit Flyers) and fragmentation and blast damage from the projectile.
Show me a machinegun in the US arsenal during the 1950s that fired a fragmentation round under 20mm specifically for the anti-aircraft role....
An Anti-Flyer Machinegun and a Machinegun in the AA role are similar but different things.
Would you care to try and visualize just how small fragments from a 20MM would be? The maximum effective range of the fragments from the 40MM grenade fired by the M79 grenade launcher or the M203 is 5 meters, and that is against an unprotected human being.
Edit Note:
1. I do have in one of my reference books a photo of a 20MM round from World War 2 exploding, along with a very similar image from the 1990s of an exploding 20MM round. I would have to scan in the photo, and then post it somewhere.
2. Tech Level 6 is cited as being 1950 equivalent, or Korean War. At that time, the standard US Light Automatic Anti-Aircraft weapon was a powered quad 0.50 caliber mount, also used in World War 2, with a computing gunsight and a rate of fire of 1800 rounds per minute. The mount was both trailer-towed, and also on a half-track for a self-propelled mount. It was backed up by a twin 40MM Bofors mount on a M24 Light Tank chassis with a rate of fire of 240 rounds per minute. For static defense, the 75MM Sky Sweeper was in development, which basically was a automatic 75MM gun firing proximity-fuzed ammunition at the rate of 45 rounds per minute. There was also a single barrel 40MM towed mount, with a rate of fire of 120 rounds per minute.
3. One problem that the US Army had with the proposed Sergeant York twin 40MM self-propelled anti-aircraft gun in the late 1970s and early 1980s was the insufficient lethality of the proximity-fuzed 40MM round, among other problems. The smallest effective proximity-fuzed round when it comes to fragmentation is around 75MM or 3 inch.
4. As for blast from a 20MM round, that is essentially nil. A 20MM rounds weighs about 4 ounces or 113 grams, with an explosive content of maybe a third of an ounce. Blast effect from projectiles is a highly over-rated damage factor, unless you are looking at High Explosive Plastic or High Explosive Squash Head rounds hitting a hard object, where you get a more concentrated blast effect.
5. Lastly, an aircraft coming under even light anti-aircraft fire, so long as the aircraft is dropping "dumb", as in unguided bombs, looses considerable accuracy, about a factor of 2, doubling the number of aircraft needed to achieve a specified result.