That makes sense Shirley, I think we're both on the same page.
When I'm picturing someone using that "hot spatula/knife" technique, I'm thinking they are meaning the old "can of hot water/wipe it off" thing...which may be way more damaging to IMBC than your slightly warmed tool technique, that's for certain!
Here's my 3 biggest tips that I use every time for getting IMBC really smooth:
1) use it when it's freshly made, that's when the consistency is perfect. After the last bit of butter & flavouring are added in the recipe, whisk it on top speed for 3 minutes then turn the mixer off and walk away for 30 minutes. Come back, switch the whisk for the paddle attachment and give it 2 more minutes on medium speed to get rid of the air bubbles and make it super smooth. It should be the consistency of soft whipped cream or soft-serve ice cream. If you have leftover cold buttercream that you want to re-use, let it come to room temperature naturally on your counter until it is soft...and whip it again to the same soft consistency. If it's a bit cold when you start to mix it again, you'll notice that it seems to split and look curdled (like cottage cheese). DON'T PANIC and don't throw it out and don't add anything like icing sugar! The butter in the icing is just too cold and it needs a good mix to return to the right temperature. Just keep the mixer running and walk away for 5 minutes. When you return, it will be smooth and uniform once again, ready to use.
2) ice really cold firm cakes, semi-frozen if need be...but not 100% frozen solid. Torte, fill & thinly crumbcoat your cake and put it in the freezer for an hour. Take it out and immediately plaster it roughly with about twice as much buttercream as you'll really need. Huge tip here....SMOOTHING CAKES IS ALL ABOUT THE REMOVAL OF EXCESS ICING AND NOT AT ALL ABOUT BUILDING IT UP AND PATCHING!! I wish someone had explained this to me when I first started in the biz. It would have saved me years of frustration at trying to get my cakes perfectly smooth. Take your plastering knife (see point #3 for this & photo attached) and run it around the cake, digging in about 1/4 inch (5 millimeters) to get off the excess icing in one or two spins of the cake turntable. Now, take your offset palette knife and draw the icing in on top of the cake from the edges, about 4-5 times all the way around. The last step is to take your straight palette knife and draw it right across the top only once or twice to remove any lines. Done! Back in the fridge.
3) Toss those cheap white pastry scrapers, they truly aren't the best. Go to the hardware store and in the plastering section you'll see cheap, stiff plastic plastering tools with handles for a buck or two (see photo attached here.) Get a few widths, like 3", 6", etc. These are awesome!! They give you super sharp sides because they don't bend at all and they are the height of your cakes, so no lines appearing as you try to ice a 5" cake with a 3" smoother by going around twice on the top or bottom. These tools do it in one stroke! Fantastic.