That employee completely changed the wording of his profile. The 15 million number is talking about the coverage of the advertising campaign, not the number of actual PC subscribers:
"Partnered with creative directors at Bethesda & Xbox Game Studios to pilot new growth ideas, making it easier to introduce Xbox Game Pass to an audience of PC players where they already were -
a campaign reach of over 15m impressions."
"A marketing impression is defined as the number of opportunities people have to view your campaign. "
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Previously the profile said:
"Partnered with creative directors at Bethesda & Xbox Game Studios to pilot new growth ideas, making it easier for over 15M PC players to enjoy Xbox Game Pass"