The optimizer uses a standard fight length - 270s for single target. Keep in mind that the optimizer is not the same as a simulator. The optimizer is our mathematical model based on simulation results.
Raidbots is a web interface for simc - there is no optimizer or calculations going on. We also have a web interface for our simulator that you can use to do simulations with different settings: Not Available
In your case here is what BiB recommends:
https://www.askmrrobot.com/wow/simulator/report/f818e77c183d45ccb18d0927d3bb9896
And here is what raidbots recommends:
https://www.askmrrobot.com/wow/simulator/report/ab188cee370d43388a1622c7c65f4652
Our model, of course, prefers the result that simulates better with our simulator.
If you change the fight to 300 seconds, then you would see Icy Propulsion pull ahead, and also you would see the trinket recommendations change a bit as well:
BiB:
https://www.askmrrobot.com/wow/simulator/report/2ffed029907744078c8019f4c1a9fbdf
Raidbots:
https://www.askmrrobot.com/wow/simulator/report/cfa24c4d415e4923a767de9c308e0d8c
The length of the fight matters a lot for a number of these effects and items, especially cooldown reduction. That is why I think Ice Bite is the better all around choice unless you are working on a particular progression boss in which you are right at that threshold where you get an extra use of Icy Veins.