The recipe gives general guidelines, not specific for BIAB mashing.
You're doing a BIAB?
Most BIAB mashes use the full brew volume in the mash/boil kettle. So for a 5 gallon batch you'd need around 7-8 gallons of water, depending on wort trapped in the grains after lautering (draining the bag), evaporation during the boil, and other brewing losses (wort left with trub in kettle, beer and trub on bottom of fermentor, etc.).
Some BIAB brewers will keep some of that water volume behind to
sparge the bag after draining (and squeezing) it after the mash. This will help retrieving some of the high gravity wort that's trapped in the (wet) grain, thus raising lauter efficiency. There are various methods for that, like pouring a gallon or so of water over the bag while it hangs or rests on a grate over the kettle, or dunking the bag in a gallon or 2 of water in a large bucket, spare kettle, or other vessel, then draining and squeezing the bag again to get as much (sweet) wort out as possible.
If you're not doing BIAB, but mash in a (cooler) mash tun or kettle, you'd typically mash with about half of the total water needed and use the other half for sparging. There are 2 main methods of sparging,
fly sparging and
batch sparging.
Use a mash/sparge calculator such as
Brew365 or one dedicated to BIAB (Brewer's Friend has one) to determine how much brewing water you'd need. That way you won't end up with too little or too much wort that's too high or too low in gravity, respectively, but exactly or at least close to what your recipe intended.
Good luck with your first all grain brew!