I disagree. There were mass casualties and it could have been much much worse - Westminster Bridge is so crowded with people. This wasn't crime, this was terrorism.
There seems to be an assumption in the article that it took a while to determine motive for the attack. That flooding the area with armed police in the aftermath was an overreaction.
That's just stupid. Once they ID'd the guy, they knew he was on their watch list. They raided his associates that night. An increased police presence is a completely appropriate response to an incident linked to an established threat, where they don't know yet (but it is possible / likely) that there were other plotters and the potential for further incidents over the coming days. Arming yourself isn't a sign of fear, it's an acknowledgement that there might be more fight coming.
We can wish that he is just a lone wolf, completely working in isolation - but the reality is that he will be part of a network somewhere, it may be irl and present in UK or it may only be online. But the attacker was part of a community with an intention to do harm. When intent becomes action, I'm glad that the security services err on the side of caution and prepare for a bigger threat until they know differently.
DH works opposite Westminster and this weeks has been really scary. He could have easily been on the bridge.