This is where the story falls apart. The "Flynn investigation" in that accusation relates to allegations that Flynn was orchestrating some sort of collusion between the Trump campaign and Russia.
Now that this whole charade has been debunked (Flynn's dealings with the Russians were perfectly legal, and were done with the knowledge and permission of the Obama administration), you're turning this "Flynn investigation" into something else entirely.
Every FBI official who has testified in this matter has said that firing Comey did absolutely nothing to stop any ongoing FBI investigation.
And since the FBI director serves as a presidential appointee anyway, the President of the United States can fire him for any damn reason at all.
Here’s the problem, and this is as good a source as any: https://www.nytimes.com/2017/12/04/us/politics/obstruction-of-justice-trump-explained.html
The interesting thing here about Flynn, is that for anything to have the slightest traction, there needs to have been a crime committed beyond Flynn lying. And there is yet to be any proof of anything. At all.
Also, Trump’s tweet was not under oath. It was a tweet. And he has a reputation for being, um, less than honest and just trolling in his tweets. Under oath he could easily say, “No, at that time I did not know Flynn was lying to the FBI as well, but when I tweeted, he had admitted to it. I was just including that info in my tweet.
This is a tempest in a teapot. And I’d say that if we were talking about Hillary and a subordinate. This is pure lunatic politics in an era of lunatic politics. It’s kinda fun to watch.