Credentials through phone # is more of a deterrent. We're also going to have an in-person coding exercise as a next step, which would be much harder to game :)
are you saying you go straight to a in-person coding exercise as the next step? seems odd, there is no phone interview between then?
Why would you want to appear better then you are at the interview? You'll get found out eventually later, and this will end up just spending your time.
There are plenty of developers/programmers who do not ace tests with rigid constraints, but are quite competent in a more relaxed setting.
That is they are not the 1% that the all hirers, but they are maybe top 20% and that is actually what most jobs require no matter the delusions of hiring managers.
Granted this particular job might be an exception but I doubt it.
In grading, the results of 6-8h of work entered in 2h would look more impressive than 3h of work in 3h, I suspect.
(Sometimes I hate that whenever someone gives me a situation, my first thought is "how can this be cheated or gamed", followed by "what controls can I put in place to prevent that, and and what costs"; it's basically automatic.)