How to run a PWM controlled fan at 100% all of the time
A fan PWM input is "just" a logic signal that turns the fan motor on and off. You only need to send a PWM signal with a high frequency if you actually want a speed between fully on and fully off, so tying it "high" is fine.

However, the article you linked says that this fan takes a TTL logic level signal. TTL "high" is at most 5 V, so you should not use 12 V as it might damage the fan's input. (Or it might be fine, depending on how the circuit is constructed — it's not like the fan has a 5V rail for the input to be too high than, which would be the usual risk.)

So, to stay within the TTL spec, use a voltage divider, and make sure the voltage is between 2.4 and 5 volts.