This mod does not change random USO generation. By the way, how do you recognize scripted vs. not scripted events???
Based on the mission, no. But in miissionScripts you can see a scripted ALIEN_PROBE_MISSION at the start of the game. Looking at the definions, such a mission is defined as a survey ship, escort and two cruisers. In my game, my radar detected only the escort and one cruiser.
Whatever you described is pretty much vanilla behavior that I have experienced many times. Once upon a time they all appear in the areas outside your radar range somewhere in other part of the globe. You don't get any money, any technology advancement and then boom: terror missions with Tasoths, Bio-drones, Reapers, Lobstermen, etc. That happened to me from time to time in vanilla games too. Nothing you can do about it. On lower difficulty levels player usually have enough time to evolve and build more bases with monthly income until attacks intensify. On SH it may be unforgiving.
I have never seen a complete lack of alien subs in my dozens of vanilla games. True, there may have often been only few UFOs in January/February (usually you get the 3 scripted ones), but zero in March as well? I don't recall a single play. By the end of March in TFTD you may even have completed all the UFO missions you need to do if you do speedrunning (except a very large lobsterman ufo).
One thing I wonder in particular is the following definion in missionScripts:
- type: invasion # begins month 6 (July)
firstMonth: 6
.. I wonder what is the default value and the behaviour (where can I find it out?). If the default TFTD value for the start of the "invasion" (whatever that implies in vanilla) is earlier, this might affect the amount of UFOs seen in this mod.
EDIT: found that in bin/standard/xcom2/missionScripts.rul the default definition is the same (but not sure if it actually does anything).