I do not believe that is true. Or at least it is not true on Chrome on Win10.
Under that OS/browser the print function will save only the posts that have been loaded to the browser. Which is typically a screen-full or so above and 2 or 3 screenfuls below wherever you’re sitting in the thread.
So this thread which has 17 posts counting mine will appear in its entirety in a print-to-PDF.
But a longer thread will not. Here’s a link to post #25 of a 50-ish post thread to play with:
I believe you’ll find that you can move around in the thread and wherever you settle you’ll get about 1 printed page above the “cursor” and about 7 below for a total of 8 or 9. No matter how long the thread is.
Again at least that’s so for Win10+Chrome. I suspect that’s true for other systems but have not checked.