Huh? That's not how it works. We don't just assume it's referring to all of history if it doesn't say "of the era". It says neither. Ambiguous.
I don't think you understand how this works. The Son is a practitioner of the dark side. The quote doesn't say "most powerful dark side practitioner sans force entities". The son is a dark side practitioner, he falls under the umbrella of the quote. The quote is literally inaccurate.
__________________ ”You presume limits to my power. There are none.”
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
__________________ "You must begin by gaining power over yourself; then another; then a group, an order, a world, a species, a group of species... finally, the galaxy itself."
- Darth Plagueis
"She may be small but she packs a punch and when shes in this mood...she goes for my danglers" - Oghren, Dragon Age
Sidious has done sh!t like unbalancing the force, causing weather changes with his hologram, bend lightsaber blades with his lightning, and match/edge the most powerful jedi of all time who himself can lift and manipulate two empire state sized ships telekenitically.
Sids scales from people who have put tears in galaxy wide light bubbles, put telekentic barriers capable of guarding entire fotresses, someone who, with help, can force the force itself to yield, who made the force intervene when he moved to telepatchically disturb everyone in the galaxy, came the closest to discovering immortality, and had learned how to manipulate and create the very buildingblocks of life itself.
Sids has suffecient feats to back up his his place in the lore as the most powerful darksider in history. That quotes are disputable don't make them worthless and with all that Sidious has accomplished, you'll have a hard time trying to dispute their validity.
Sidious's mastery of a lightsaber, being the supreme duelistup untill his time, makes sidious a far greater combatant than Valkorian.