Ay Op Matey peeps...... Long time!
Can't say I have ever seen any carbide knives...... I assume they are a knife with a carbide tip.... so the cutting edge will last longer than a standard worked steel blade.
Kind of like a carbide tip cutting disc.... you get different tip finishes which will last longer but will cost more and more, from steel to hardened steel to carbide tip to diamond tip......
depends on what you cut, how long the blade lasts and how much the blade costs......
Standard stanley knives are cheap as chips, work fine on boards that can be cut like that.... if a carbide tip blade lasts longer - see how much longer verse the cost difference.
Hardi boards have specific cutting tools - I have tried to cut using a stanley knife.... hard work! Their specific tool does the job.