As MES have done "an investigation" and as Horner has not disputed MES's findings (which by now I believe he has had sufficient time to do) then I have a tendency to lean towards the MES "investigation" results.
Now who is taking sides?
Seriously though, that is like saying once guilty then always guilty.
This message board would suffer a major melt down if that were the case.
MES' findings stated the actual ECU does not have a problem, but the data program does. It would be pretty much like my laptop repair centre saying 'your laptop is in fine shape but the operating system doesn't work.' Add to that fact, they actually issue an apology to Red Bull in the exact statement. My assumption is naturally to trust that Horner got it right, read the apology and saw it as case closed and no need to further comment. (lbennie makes this point very well.)
Of course, I may be wrong. And I do not deny I am taking sides, in fact the precise opposite side of your point-of-view. But it is just an opinion. However stating Horner shot himself in his foot seems to assume that he is wrong as a fact
so I am just stating the opposite may be true as well.
And no, once guilty does not mean always guilty. But then again, most people tend not to believe in the boy who cried wolf, so precedence does play a part in making judgement on any given event.
While all teams are, at times, economical with the truth to the press, I doubt this is one of those occasions. Lying in this instance is the sort of thing that could snowball in to serious repercussions, with lawyers getting involved or putting their exclusive contract in doubt by pissing off the FIA.
Lying and omission of facts are very different things. MES never outright claimed they had no part to play in the failure at all, just that the specific ECU was not the cause. But the programming or other ECU related equipment may have been flawed, thus the apology.