"That's not completely true, Mr. President." Bobstays, (on boats that have them) can also be used to adjust the forestay tension.
I would suggest that this is near total misinformation since bobstays should almost never be used to adjust forestay tension. I say 'almost never' because there is one design that does allow this. Traditional English cutters had a "reefing bowprit" which could be retacted back onto the deck, and was set up with tackles. Here the trust on the heel of the bowsprit could be used to tension the forestay, but I do not believe even here there was a tackle on the bobstay which would allow the bobstay to impact forestay tension.
The much more normal condition is that Bobstays are typically engineered with larger diameter wire rope (or solid rod these days) than the forestay, even though they are much shorter in length and are often on more efficient angles than forestays, with the fullest intent that there will be very minimal amounts of stretch in the bobstay when loaded and therefore almost no vertical movement at the tip of the bowsprit.
In most cases, over or under tensioning the bobstay would place unnecessary strains on its mounting points and would also have minimal impact on the forestay. On a reasonably sized boat, properly tensioning a forestay typically requires several inches of take up. Few, if any designs permit this kind of movement as way to get the required amount of tensioning through moving the tip of the bowsprit up and down.
As mentioned above, more typically on modern rigs with bowsprits and furlers, the backstay and lowers are eased allowing the mast to sag forward until the forestay can be attached. Then the backstay is retensioned to tension the forestay. Some furling systems do permit some small amounts of adjustment which intended to allow the length of the forestay to be adjusted for stretch, but which does not work very well for tensioning purposes.