Glad to see a fellow IT Consultant moving into the Strategy Consulting space :)
So the answer here lies in between. You need to be familair with M&A AND IT.
Now, unfortunately, there aren't many resources out there for the intersect of the two. So, you'll have to learn about the various types of M&A cases (PE firm acquisition, merger, divestiture, etc.) and you'll have to make sure you're well versed in the various current IT topics (digital transformation, cloud, agile as scale, etc.). You really need to do both.
Now, it's "funny" you bring up this questions as I've just published a case that actually is at the intersect of IT and M&A! You can check it out here:
Feel free to reach out if you have any further questions!