It isn't negligible on the timescales we care about. Skimming through the graphs in [0], at 900km we're talking 100+ years of orbital lifetime. And even if most fragments were to disappear sooner[1] - giving them a generous 20 years - that's still 20 years of dealing with a swarm of untrackable hypervelocity bullets coming at you from random directions. While space is big, it's not that big in LEO, and any such bullet will, upon impact, spawn more bullets.

I don't think we'll have a catastrophic cascade leading to Kessler syndrom even if these two satellites collide today, but few more such events might be enough.

EDIT:

Also, the collision will be off-center and, eyeballing the orbital diagrams, some 15 degrees shy of being head-on. That means you can expect most of the both satellites not being directly involved in the collision, but instead gaining spin and breaking apart under stress. Those surviving pieces will then keep most of their orbital velocity and heading, but scatter like buckshot, each piece on slightly different orbit, migrating further over time.

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[0] - https://www.spaceacademy.net.au/watch/debris/orblife.htm

[1] - Orbital decay due to atmospheric drag is a function of mass to area ratio. I'm not sure whether the debris from a hypervelocity collision tends to be more like irregular, high-drag shatter, or like low-drag spheres of superheated metal that cooled off radiatively.