By this logic then Servant who is summoned normally and not nerfed like Artoria summoned by Shirou cannot get stronger by Command Spell, when it was canon it can strengthen Servant.
Command Seals don't power up Servants like that, all it does is give them more Magical Energy without draining it from their reservoirs or further add onto that.
I think at some point in F/SN it did state that more magical energy=Servant get stronger.
There is no point in F/SN where that's stated, if anything, the opposite is stated because Servants get weaker having less Magical Energy because they can't depend on their busted Reinforcement for damage, easier to kill as their spiritual bodies are weakened for easier access to their Spirit Core and can't use their NPs for fear of instantly vanishing.
Grail give magical energy, so shouldn't Servant should get stronger universally then?
No. Because Servants have caps in their Spirit Origins and cannot go above that maximum, not even in FGO(Morgan and Altera explicitly have lines that show they are not able to get even remotely close to their living selves in terms of power). You are misunderstanding having practically infinite Magical Energy for "strength" when it's more the equivalent of being able to fire off extremely BS Kamehamehas without any cost.
Kriemhild in Traum having 2 Grails is noted to be really strong because of it. Not that they all get the same upgrade just because they get the same grail support, obviously some that have Mana Burst benefits more, but it would still give every Servants more power by simply having grail.
Again, Kriemhild literally got the equivalent of a Spirit Origin Ascension without having one. Traum stated that having 2 of their specialized Grails was her limit and she can get no stronger due to it. It also only affects the Kings of the Realms and no one else benefits from it at all. The infinite Magical Energy only works as a "boost" to power otherwise when it comes to equal Skills like Mana Burst(since you are using that extra Magical Energy as an amp) or if your Noble Phantasm has extra applications you can bypass(Like with Tametomo using 7 Grails to bypass a fuckton of the extra shots he needs with his NP to only require 2 for the insane shenanigans we see in his event).