Nope - it is a deliberate misuse. You have two different definitions of "they" here, and disingenuously try to blur them. Please cease and desist, as this only makes you look like a total idiot.
I don't recall you being made a part of the moderator staff here, Timo. I'll post what I like. If you don't agree, that's fine, but I won't have you dictating what I am or am not allowed to argue. It's not your call to make, as much as you appear to believe otherwise.
What underestimation are you talking about? The amount of forces sent to Alpha was in no way decided by "them", but by circumstances.
The Dominion believed they'd be able to defeat the Federation in a matter of weeks following the original signing of their agreement with Cardassia. In the months that followed, weekly convoys of ships, troops, and equipment arrived from the Gamma Quadrant to bolster their military forces within Cardassian space.
It's not clear why the troop movements happened on a weekly basis, but given the Dominion's penchant for assembling troops and building ships quickly (something supported numerous times on screen), it's surprising that they would not endeavor to send either more frequent reinforcements or larger convoys to Cardassia: their remaining military presence in the GQ would not have suffered for the transfers.
Weyoun tells Sisko that the convoys are an attempt by the Dominion to reassure the Cardassian people that their allies will adequately support them given the planet's economic woes and the military devastation wrought by the war with the Klingons. It's not known if his remarks are genuine given that they followed Sisko's statement that he would not remove the mines being placed at the mouth of the wormhole, but if we're to believe that they're accurate, it would seem that the Dominion's strategy was influenced at least in part by the Cardassian's cries for help rather than by the perceived threat the Federation and its allies represented.
That the Dominion was later thwarted by circumstances beyond their control is something that they could not have foreseen, but as they'd already underestimated the Federation by not arranging for additional soldiers and vessels, they have only themselves to blame for being caught unprepared. Their remaining Alpha Quadrant forces may have been enough to defeat the Federation had the Romulans not entered the war, but we'll never know for sure.
Whatever the case may be, I'm not placing an asterisk next to the Dominion's name because they didn't have enough soldiers fighting for them.