You say 'comprised of their employees' - who determines this? You need a secret vote at least, or some kind of voluntary indication of support. You need to know people aren't being coerced.
That said, it is also common for acquisitions to be structured as a transaction in which all employees are terminated by the acquired organization and hired by the new organization. This effectively "resets" the process and may require employees to hold a new election. However, employers can always recognize a voluntarily and skip the NLRB process which would make recognition mandatory. It seems that Microsoft has indicated their willingness to do so.
The concept of a normalized card-check-only process is generally favored by unions because the NLRB election process is widely viewed as excessively favoring employers. However, it has never passed congress. That is, the "card check" as it is often discussed doesn't actually exist as the legislation proposing it has never passed. The existing mechanism is a fairly narrow exception that mostly exists for employer convenience.
A union is by definition a group of employees. I guess you're talking about, maybe, representatives of a national union coming to an employer without there first being a union vote within...?
That's not remotely like what's being discussed here. What this situation is, and what's being talked about, are a union formed within the company (whether or not it's affiliated with a national union) and whether the employer has to recognize it and bargain collectively.
I can't even fathom the idea of a union with no members at a given company or organization "just showing up" and trying to get the management to bargain with them on their workers' behalf. It's just utterly nonsensical.
That's how many unions work.
For example look at what is happening with Amazon warehouses - existing external unions turn up and try to recruit workers.
You also see it for example in the UK where unions have a political party and try to use that to change labour laws through normal government elections.
Whether or not that's nonsensical is a matter of opinion!