Everyone so far is asking the wrong question. The question is Why can all Guardian modules be fitted with the Anti Thargoid field? My speculation is that the current mechanic of Titan gameplay continues past the the last Titan in Human Space. As the FSD booster can also be modded, then that suggests that we need to travel to an area where the field is everywhere.
This may mean access to the formerly permit locked areas where Thargoids are the dominant life forms

. The time invested in the gameplay loop of Titans, Spires etc. if it stopped completely would be the end of what has become for some their main gameplay. I expect it will continue in some form or other, with the above speculation being an extension in taking the war to Thargoids scenario.