Yeah it's all opinion so no worries.

Now I don't know what was actually going through QT's mind at that moment but just trying to break down the clip a little more.

While QT didn't have vision mere moments before the Malphite actually ulted as wildturtle did in his case, Malphite was still seen right at the start of the video with more or less full health and a third of mana left, so there wasn't an immediate need for him to back just yet. A few seconds before the flash-ult happened, QT looked at the map, you could see Graves was quite far back (and a few moments before it all happened he seems to look down again, you can see that both morg and graves are walking back, but still no malphite). So it's highly likely that QT knew that graves wasn't going to be involved in any immediate fight, and he may have also known about morgana too (can't tell what he looks at the second time) but he hadn't seen malphite far away yet so still had to treat him as a threat.

I quickly checked the replay (replay.gg, most ranked games are on there) and QT had to cancel his ult as Malphite threatened the ult with a protobelt cast, so it was clear that Malphite had ult. Now, there are also a couple of more interactions that may have given this play away too. Firstly, you can see from QT's perspective that they dropped a ward so he knew that they had full vision of them in that area. Secondly, you can see that initially Syndra is moving back with the rest of her team and as she reaches the tower ruins you see she turns, (checking the replay you can see that this is the point malphite starts walking into flash-ult range and a ping onto QT so Syndra positions to back him up) to defend seemingly on her own against a blitzcrank+fed jhin? That might have been a red signil that she wasn't as alone as she looked. And thirdly, as I mentioned it's a lone Syndra against a fed Jhin and a Blitz, chances are a hook means her life, and it seems that Blitz was looking to make this play in the way that he's moving up. Malphite would have countered this beautifully if he hit the ult, as we can see near the end of the clip Syndra almost kills QT, so a Malphite E+ult along with Syndra's burst probably would have meant his life.

Then there is the amount of time that both Wildturtle and QT had to react and flash away. In both instances the Malphite flashed into vision from fog of war, but if you slow down/pause the clips, you can see that the Malphite in the Wildturtle clip flashed in much closer to him that the Malphite in the QT clip, meaning that Wildturtle had less time to react than QT as the Malphite in QT's clip had further to travel, albeit a couple of teemos in length longer.

Then of course, as someone already said, you can see that QT had his mouse hovered behind him for a fair amount of time readying for the flash.

Sure, maybe I've over-analysed the hell out of the clip and it was simply just a really good reaction, so while credit is due for actually being able to do so, there is still a huge element of expectancy involved in higher levels of play that we should really be expecting them to do.