Alonso should be acting like a professional, but everyone has their breaking point, nobody thought that the paring of Honda engines and a McLaren chassis would be so poor, it was always going to take time a couple of seasons, but there has been no improvement at all, Alonso will have insider knowledge of what is going on behind the scenes more than any outsider will know about, Alonso's problem is that he has nowhere to go, he won't go to Mercedes because of Hamilton, he wouldn't be able to go back to Ferrari, Red Bull maybe if Verstappen can extract himself from the team, but then where does he go to as Mercedes and Ferrari are out of the question, and anywhere else will be a step down the grid really.
How many engines is Alonso's car on now, they only have so many to use, is it worth the wear on the engine for a future race if you are going to finish last, Spa is a power and speed circuit which is not suited to the power the Honda can give, why not save it for a race where speed and power is not so important?
Alonso is just venting, and to be honest the bosses at McLaren are not hiding their frustration very well either.
I am saying this as somebody that doesn't like Alonso very much due to the various controversies.