A terrorist attack isn't defined by the number, competence, efficacy or colour of the perpetrators. It's defined by the intent of the attack.
If that intent was to disrupt or harm the government or the apparatus of the state then it can be deemed a terrorist attack.
If that intent was to diminish people's sense of safety within the borders of their own country against an outside threat then it can be deemed a terrorist attack.
If that intent is to stir up social and cultural divisions that compromise people's sense of safety from an threat from a "them" within the borders of their own country then it can be deemed a terrorist attack.
Today's attack at Westminster ticks all three of those boxes and, honestly, the death toll is practically incidental. Even if no one had died today, the media frenzy would still have stirred up a very similar reaction. Also, "one guy with a knife"? Surely on this forum the other weapon used should not be difficult to recognise.
And please don't buy into this rightwing tabloid shit-stirring language where the ist is dropped from terrorist attack. It just feeds into their bullshit moral outrage that they manufacture to sell papers and advertising space.