Maybe, but maybe they just don't like the sound effects and wished there was music? ;) I notice there are several comments on the Youtube video for this game complaining there is no music.... ;)
However, as I said before, the quality of the sound effects is not the issue for me personally - I would NEVER play a shmup that does not have music - to me, music in a shmup is the same as shooting, power ups and boss fights - a necessary and important part of the experience.
Almost all C64 shmups have music - certainly all the best ones - and the soundtracks in a good shmup are celebrated as much or more than any other part of the game. To have a shmup without music, to me, is to have a half finished game that is not worth playing.
I have never come across a C64 game with music that overrides TWO channels to sound effects - ONE yes, many do this - you lose a single track of music playback when a sound effect plays (R-Type, Katakis, Warhawk etc), which I do not like personally and if the option is there I will always switch off the sound effects altogether to prevent disruption to the soundtrack.
Of course it is the designers choice to make any game any way they want, and that's fine. However if the designer is making the game for their own benefit then it doesn't matter, but if the designer wants people to support his product he needs to make a game THEY want to play and support. I have purchased literally hundreds of games on Itch, Protovision, Psytronik, Double Sided Games etc - in fact, I try and purchase almost every C64, ZX Spectrum, Vic 20, Atari 8 bit etc game that is released to support further releases. I purchase shmups, platformers, puzzlers, text adventures etc. However, I draw the line at a half finished shmup with no soundtrack. But again, that's just my opinion - if you like a barren stark sounding shmup with no soundtrack then enjoy.
As for many C64 sshmups not having music - I DEFINITELY disagree. If you made a list of either ALL the C64 scrolling shmups, or even a list of the top 20 shmups on the C64 I can guarantee you that the vast majority of them have a soundtrack. Armalyte is one of the few that comes to mind that doesn't, and I always thought it was a boring bland shmup because of it - but again, that's just my opinion.
I just think it is a shame as the game looks quite decent..... it just doesn't SOUND decent, but could have done. Still, don't let me stop you from enjoying it.