"that is evidence of an intended cover up"
No it's not.
You're conflating the fact that this is mostly a political issue, not a legal one, and that even impeachment is mostly a political process.
Every politician on this earth will move to obscure information that is regularly private, so as to not make it available for their opponents as fodder.
In the management of 'perception' it's not 'evidence' it's just fodder for spin, talking points etc..
If Trump was legally required to submit the letter, and did not, this would be a problem, but otherwise, it's rational for him to want to hold it back as the story broke.
Once the team looked at the materials, surely they're going to let go of that which they feel helps their case publicly, which is perfectly normal.
I don't like Trump at all - but this specific transcript and even the handling of it - is a complete nothing burger.
I'm completely skeptical of all of the press because of their constant misrepresentation of all of these issues.
I feel slightly more enlightened having actually read the transcript myself.
Apparently, the White House instructed the Pentagon to withhold aid, which is illegal, and that's cause for concern, but given the total lack of any material exchange in this transcript, Congress will have to provide materials that show the direct quid pro quo.
I basically hate Donald Trump, but that doesn't mean I want him committed to bad justice.