The ad, which you can read on Slack's blog, harkens back to a similarly passive-aggressive page-long letter Apple placed in the Wall Street Journal, "welcoming" IBM to the PC market in 1981. Make no mistake, it is some seriously sly advertising. While ostensibly a friendly greeting to Slack's newest competitor, it goes on to make a point-by-point case for why Slack has been (and will remain to be) the most popular team collaboration tool on the block.
Microsoft is the second major tech company to dip its toe into the collaborative messaging space recently. It joins Facebook and its Workplace service in an attempt to cash in on the growing number of businesses who are eschewing stodgy old email in favor of real-time chat messaging. It's a trend that Slack was first on the scene for, and it's since become a runaway success.