No this isn't true. I think this is where a lot of people are getting confused. It's a exponentially weighted average, about 90% over that time frame. But all the data you have ever put in, contributes to it, just the further you go back the less weight it carries.
It takes a long time, but eventually for the most part your CTL will just become your TSS average for the previous 42 days.
But imagine you started at zero, the way it works is let's say you did 100 TSS a day. After 42 days your CTL would be 63.7. It would take you 316 days to finally reach a rounded figure of 100 CTL. After that, your choices are do more intensity, do longer durations, or both. If you stick to 100 TSS at that point, in simplistic terms , then you have plataued.
I've mentioned before, this is why the second year I trained like this, the gains have been harder to get. You have to be creative and try and do more. But still, essentially every session ever still contributes, it's just the stuff i did 2 years ago is now minuscule in the calculation.
I will also add, it's probably only useful if you are trying to compare like for like. If you are really mixing up your training outside of what I am doing, rTSS probably isn't going to tell you the whole story anyway. The consistency of it (much like with power) seems to really work around the sub-threshold level. Of course, I have said before probably easy runs are over represented in rTSS. But again, assuming you roughly run them at the same % of intensity relative to your current fitness you input, it'll stay consistently over represented in time, so doesn't really matter if our intention is just to use it as a practical tool (which is all I care about). I'll happily leave the science to the much smarter guys than me.
It's likely why the graph I shared on Strava fit so neatly, in terms of load versus performance, because my sessions are still for the most part comparible in intensity going right back (with a few changes to sneak in extra load) but also a decent invease in overall training time.
In absolute simple terms , my daily average per session a year ago was 60 TSS per week and now it's 70. There's probably no real suprise that I'm fitter now, because I'm doing the same thing with a sprinkle of pushing the limits of sub threshold intensity, or just more or the same but longer. How you make up that increase is for you to work out, but that comes with experience if training like this.