Using an iPhone while promoting Google Pixel is directly damaging to the Google Pixel brand.
This employee made a significant mistake.
It may not be that easy to undo; the Internet has a long memory.
Years later, people will still be having conversations like, "LOL, you're not thinking of buying a Pixel? Did you know Google themselves send tweets from an iPhone to promote that thing? Incredible, but true!"
What Google can do here is use their dominance in the search space to make it hard to search for this incident.
It's not a cult. You don't gotta buy Google stuff.
Now if it was an apple employee...? More surprising but like, not worth an article.