Eh, personally (from experience, albeit I'm a fair skeptic of CAT appeals) I think it would be incredibly, incredibly difficult in this case to try and convince CAT that the CMA has erred on a judicial review basis on its market definition. It was significantly easier in CTM because as you mention, CTM was not a merger case but rather a TFEU/Competition Act case where the underpinning review jurisdiction of CAT is that of a merits jurisdiction on both law and fact - in other words, it could arrive at its own conclusions on any issues in both law and fact and didn't have to apply a judicial review standard to those issues. It could also in that case effectively determine whether the underpinning decision was right or wrong on its merits even if the CMA got the market definition wrong, so it was more than willing to engage in that substantive analysis. In this case, it will need to essentially ask the question (if Microsoft of course raise it as a ground on appeal) whether the market definition choice was on a judicial review standard irrational, or otherwise blatantly false/not materially sound (I doubt they'd go for illegal/procedural impropriety etc!).
Noting how high a bar irrationality is in any case (in this context, a decision so unreasonable that no other reasonable regulator could have arrived at the same conclusion - a ridiculously tough ask prima facie given the two other primary regulators both identified potential competition concerns in cloud game streaming!), as CAT and the CMA have said previously, market definition in the context of mergers is an analytical tool and, flowing from that, you can then appropriately consider relevant theories of harm and SLCs. By the same token, in any given merger you may have multiple, overlapping market definitions (some clear markets, others more obtuse but no less relevant), but the point is the market definitions don't necessarily need to be rigidly drawn for you to then evaluate theories of harm - they can be used as a framework to explore actual competition issues between relevant parties where you have (and what the CMA have, at length, tried to justify in this case) dynamic competition in a market. That's uncontroversial as far as legal principle goes (noting that market definition is not really a rigid legal principle - it's malleable to the context to an extent).
All of which is to say, what the CMA have done in their various filings is try and evidence the above i.e. that drawing very rigid market definitions is difficult and so these are a framework tool to try and then drill down, in particular, to dynamic competition (and potential SLCs) between firms operating in a particularly defined market (in this case, supply of cloud gaming services). CAT have said before that traditional tools of analysis (including, amongst other things, market definition) are not as useful/determinative as they might otherwise be when analysing specifically this (dynamic competition) and in Meta expressly endorsed the idea that weakening of dynamic competition (again, which the CMA in their various reports have at length tried to assert forms the bedrock of their analysis) is absolutely sufficient to then result in a finding of an SLC and the ramifications beyond that point.
It's also probably worth noting, again as CAT noted in Meta (and this is broadly high-level), that asking it on a judicial review standard to effectively quash a decision of the CMA on the basis of getting a market definition "wrong" is difficult because a) the CMA have a wide margin of error in merger cases under the EA2002 to define markets and b) as above, market definition is just an analytical tool to see whether there is an SLC - it's not a rigid barometer that determines whether you (i.e a regulator) get the assessment right or wrong.
Sorry for the small essay (had a lunch break and it's a really interesting question/theory which I've been thinking about for a while!). I'd also note I'm not making any value judgments about the CMA's framing of the market, whether they're right or wrong etc - rather just outlining what their argument is and why I think it's probably very difficult for Microsoft to succeed on this point.