Even if the Space Marines could get every single one of their chapters to actually work together instead of mildly distrustin each other at best to shootin each other on sight at worst they'd still be horribly horribly outnumbered by the IG.
Really the worst part about that engagement is exactly that bit. While there are a decent number of Space Marines, they don't all work together. Some of them actively hate each other. Some actively shoot at each other. The IG doesn't have that problem. Sure their commanders will send you to your death and sure their commanders have absolutely no qualms about shootin their own men, but at the end of the day they still all have the same goal and work together.
Really the worst part about that engagement is exactly that bit. While there are a decent number of Space Marines, they don't all work together. Some of them actively hate each other. Some actively shoot at each other. The IG doesn't have that problem. Sure their commanders will send you to your death and sure their commanders have absolutely no qualms about shootin their own men, but at the end of the day they still all have the same goal and work together.