I agree with you. Infact to further support your claim, Man UTD in the last three years preseason games have been exceptional, but suffer in the league so it will not be okay to start praising them yet till they can be able to repeat this performance when the season starts.
The format in which they are playing this is like a series that at the end would result in an award of sorts so that's the thing with Man UTD, when they know that what they are playing for, a prize is attached, you'd see them coming all out and giving their best. I just hope that they can put this same mindset into their gameplay once season starts cause it doesn't sit well that a big team like them are struggling in the lower part of the table.
We can't trust them just yet because their is nothing at stake, I have seen so many teams do well during preseason and when the league title race start they start fumbling, besides by looking at the game you will see that this Bournemouth team is not playing with the same intensity as the are used to, because they knows that nothing is as stake, so they don't want to risk getting injured. I know that some football fan might think otherwise that Manchester united are picking up but as for me, they are still in Disney land because i wouldn't bet on them just yet, because I still don't trust them yet, let the season start first, after the first leg, before the reverse fixture will start, their performance then will determine if truly they are back to the Manchester united that can compete or not, but if Manchester united do not spend more money on incoming transfer more than they have already done, they might pays for it because the rest of other top sides are not playing.