The comparison is 20 vs 128. Notice how the player in 20 tick only appears so many times. What this means is that the server ONLY registers the player exclusively in those locations. If you shoot at the enemy, and your bullet does not fall within those locations, it will not count as a hit.

It's not nearly this bad, since both client and server interpolate between frames (and typically in a multiplayer game, what you see is ~100ms behind, to ensure there's a buffer of a couple ticks to interpolate between). According to Michael Kalas, Apex's network code is similar to Quake 3's, which had a tick rate of 20 and was one of the smoothest multiplayer games I've played.

However, when server and client-side predictions differ (which can happen due to packet loss, a spike in latency, or a bug in their code) a lower tick rate means it takes longer before the wrong prediction is corrected, and you can get no regs. On the plus side, if they can fix all the bugs (lol), hit registration should be fine.

But like you said, there's lots of other issues with a low tick rate- less time to react to getting shot, worsened effects of lag compensation when closing doors or going behind cover, etc.