Yes, your understanding is correct. The "technique" technically only refers to the the positioning of the linemen before the ball is snapped. This refers to all defensive linemen, but is typically only used when referring to defensive tackles. Also, when someone is referring to a "5 technique lineman" you can typically just replace that with a "3-4 lineman". Likewise, when some is referring to a "3 technique lineman" you can typically just replace that with a "4-3 lineman". Now, this isn't always the case, but it can be helpful when trying to communicate with someone who is using these terms.
Teams that play primarily 3-4 defenses will be most interested in players that play the 5 technique because, as the article you referenced states, in the "true 3-4" defensive front you have one defensive linemen playing the 0 or 1 technique and two defensive linemen playing the 5 technique. These linemen will typically play the two-gap.
Teams that play primarily 4-3 defenses will be most interested in players that play the 3 technique because in the "true 4-3" you will have one defensive linemen playing the 3 technique and one defensive linemen playing the 1 technique while the two defensive ends will play outside the line either in a 4 technique or in a wide nine set. The two defensive tackles will typically switch between playing the 1 and the 3 technique throughout the game which is why it is important to have two defensive tackles that are good at playing the 3 technique. These linemen will typically play the one-gap.
While this "technique" terminology only technically refers to the positioning of the players on the field, it is important to note that in each of these alignments are typically used to do specific things (containing the run from the 3, rushing the passer from the 9, etc...). So, offensive linemen (and spectators) can typically determine what the defense is trying to do just by looking at the alignment of the defensive line.
