One thing people here don't seem to grasp is the fact that for Electronic Arts, "install base" is not the same as "consoles sold".
Install base for EA gives them an estimate of how many customers are available to sell software to. We can safely assume that EA thinks (and I think that this is a reasonable assumption), that XBox1 X sales went almost entirely to Old-XBox1 owners. Hence while the number of consoles sold increases, the install base does not. They also neglect the "trickle down" effect of old consoles sold by new X1X owners into the secondary market over time (generating an increasing install base).
In the end Electronic Arts' "install base" logically is lower than "actual consoles sold", likely by a few millions. So unfortunately, this EA statement is no secret path to get to the "precise" sell-through numbers for XBoxes.
It's much more likely that it is simply a bad estimate. There are two ways how EA can estimate:
1. Consoles sold - Here the numbers don't add up because ZhugeEX said that Xbox One was "around 35m" by the end of 2017. The real number is around 108-109m, therefore EA made a bad estimate.
2. Installed base as you say, so PS4 Pro and Xbox One X sales do not count towards the total - While this could explain the estimate of 103m by the end of 2017 (PS4 below 70m, XB1 ~34m for a total of 103m), it leaves the question how PS4 and XB1 are supposed to combine for 27m units in calendar year 2018. That's a number these two consoles cannot possibly reach if PS4 Pro and Xbox One X sales don't count towards installed base, so once again, the conclusion has to be that EA made a bad estimate.