I didn’t read anything on the site, or about the graph, but apparently It’s saying:
for momentary peaks, you should have enough power for +20db.
(more wpc needed for speakers with low sensitivity, and watch for low impedance dips)
and, it is showing, a 13" woofer would go down to 43hz (that’s why people add subs for lower frequencies), and a 7" would go down to 118hz (even more need for sub(s).
Keep in mind, there are two of each 13" or 7" in a stereo pair.
What SPL ’in your space’ reaches your ears ’at your listening location’ (also, how relative to other frequencies) can be determined with an inexpensive SPL meter, and a CD with test tones.
Make sure the SPL meter has a bottom hole for tripod mount, so you can set it at ear level in your spot.
this one has a tripod hole, many do not
if you know how, you can download and make your own cd of test tones
https://www.wavecor.co.uk/testdisk.html
I use this 'Amazing Bytes' CD, but it has gotten expensive
https://www.discogs.com/release/7290000-Various-Amazing-Bytes