Harry did ice magic before, yes... But iirc before he became Winter Knight his ice was always a product of him performing his old fuego spell. He had water, from which he took the energy to fuel his fire, letting a huge flame go up in the sky, thus turning the water to ice.As Winter Knight he somehow simply creates ice, or so it seems. There is no exothermic energy released in form of fire, no water surface needed.
Also the ice below the pursuing fetch. I think all wizards can probably do all sorts of magic, but are not GOOD at many forms. Harry did veils before Molly, but wasn't very GOOD at them, though he got better when he analyzed the practice a bit. Now, maybe he funnels ice from elsewhere - the cold of, say, frozen gas giants - the chill of Jupiter. Maybe he can link to Mouse's ice world - Mouse was an ice demon, right? Mouse cheats by living with a wizard - he channels his power source thru Harry.
I think of it in a gaming sense like each school (fire, ice, earth, mind) is different. Harry was level 14 fire, 2 in ice, 1 in the others. Really nasty at one. But say the Knight mantle confers 10 levels in the right school. Fix is now level 10. But Harry WAS a 2, so now he is a 12. So he is nearly as good with ice as he is fire, so he has options.
You and a buddy have to kill 2 men at 50 yards. You have 2 rifles, and 2 compound bows. You've never fired either. You reach for a rifle. Your buddy never fired a gun, but he's a bowhunter. he reaches for his familiar weapon, the bow. It's not AS good, given equal skills, but with his greater bow skill, he thinks he will be better with it. Harry is the bowhunter, fire is the bow. He was good with it, so he uses it when another skill might work better - but it is a skill he does not have.
And also no soulfire.
No, that's mine. I note fire is usually a destructive force, Harry mentions that is why wizards use it. Ice can be made into things - sculptures, igloos, etc. My idea was Harry might be able to imbue an ice structure with Soulfire to reinforce his shield temporarily. Sort of the ice is Kevlar - the Soulfire is the armor in your plate carrier, because Kevlar doesn't stop that .30-06. So some small critical area is reinforced
Maybe he was always able to do that and just because he is the knight now, makes him believe he must be able to create ice, so he can.
Just like he believes to be able to create fire.
I don't think he CREATES. I think he CHANNELS. Like Binder, calling forth the Gray Men. He calls ectoplasm from Nevernever, he does not create matter. Fire? The Sun's corona ? or even plasma if you are good enough. Cold? South Pole? Mouse's ice world?
Carlos for example is said to have an affinity to water magic. Does that mean he can create water? We never see him do more water magic than his disintegrating shield. But this I think removes water, instead of adding.
maybe that is what made the frictionless dust in the deeps duel? He yanked water out?