The rules for this game are very vague and I suggest they are updated. There is no clock in this game so you can basically kill enemies infinitely on any given stage. There are multiple places on each stage to access a bonus stage. You can only access each of these once per stage unless you take a death. The current record for this game is 235,900 and there are only two possible ways I see how a score this high can be achieved.
1. Farming enemies near a place where there are two bonus stages and then getting an extra life. As far as I can tell you get an extra life every 20,000. This means you would kill 10k worth of enemies and then max out both bonus stages and then die on purpose. I believe this could be done infinitely but it is mostly moot because of option 2.
2. You can find many places in the game where you can simply stand in one spot and just kill enemies as they approach you. Only two enemies can be on the screen at once so it makes it very easy to do this. You can farm essentially forever with little to no threat of death. The game even gives you health refills as you kill enemies.
So this begs the question? How much leeching is too much? With no videos of the current records it is pretty difficult to determine an appropriate amount of leeching but with almost no other way to score points it basically comes down to leeching slightly more than the previous guy did in my opinion. The game rules could be set to marathon settings and then it would simply come down to who was willing to play the game the longest or a per stage leeching maximum could be set and then it would come down to who could get full points on all the bonus stages and strategically use there lives.
I want to break this record and in its current form is going to be me just leeching 235,900 and then beating the game after that to barely break the score. This does not seem like a twin galaxies style path to me.