IIRC it's just reduced by a factor of 1/2 rather than the projectile armor damage reduction formula.
Common mistake, but no, that's not how it works.
Beam weapons use the same formula for armor damage reduction as projectile weapons:
damage dealt = damage-of-hit * hit-strength / ( hit-strength + armor )
For projectile weapons, hit strength is just the damage of the hit.
For beam weapons, hit strength is 1/2 the beam's raw DPS (adjusted by damage type as usual).
So, the HIL does 500 DPS, x1/2 from the beam hit strength calculation, x2 because it's HE, for a hit strength of 500.
Where this gets complicated is burst beam weapons - the "raw DPS" I mentioned above is not the DPS listed in the codex, but the DPS value you can find in the weapons.csv file; essentially, it's based off of how much DPS the gun would do if it were firing continuously. Which is why the Tachyon Lance is so ruinous against armor: its raw DPS is 1500, which gives it a hit strength of 750.
And just for reference, here are the hit strength values for vanilla burst beams (technically as of a few Starsector versions ago, but I don't think they've been changed recently.)
Burst PD Laser: 175
Phase Lance: 500
Heavy Burst Laser: 200
Guardian PD System: 500
Tachyon Lance: 750