Speaking from personal experience, that's not always true.

When the company I worked for was acquired for team talent, they interviewed all but a handful of the engineers. Around half were given full time offers. Another group were given fixed-term offers and the remaining engineers didn't make it in.

I was given a fixed-term offer. Several months and many interviews later, I made it in as a full time employee. I was never outright rejected, but due to circumstances involving my contract and project work, I probably took around 9-12 Google interviews over that time.

They aren't fun. But in the end, I'd say it was worth it.

I doubt this is always true. I've been involved in acqui{hire,sition} due diligence for a number of startup at a couple of different (large) internet companies. My experience has been that the terms can vary quite a bit, both in the set of folks given an offer (all, all or none, some) and in valuation relative to average developer comp at the acquirer.

After reading your comment multiple times, I have no idea what you said.