ImClarky has the rule established for grounding but this has nothing to do with your question.
Simply either Wikipedia or your interpretation of Wikipedia is incorrect. A receiver who steps out of bounds (let's clearly talk about NFL since this varies in high school and college) IS AN ELIGIBLE RECEIVER.
A receiver stepping out of bounds does not become ineligible. The easier way to think about this is if this were true and the receiver stepped out and was more than 1 yard downfield he would be an illegal man downfield.
He is eligible and if he touches the ball first (without any tip) then and only then is it an illegal touch.
Think about this too. A lineman can have an illegal touch if they are the first to touch a forward pass. That is an automatic penalty because they are an ineligible receiver.
An eligible receiver who steps out of bounds on has three outcomes:
They are not the first to touch the pass therefore they play as normal.
They are the first to touch a pass and do not catch it. Then it is illegal touching yet the outcome is the same as the incomplete pass. The reason this isn't a "dead" foul like it used to be is that it give the defense the opportunity to play the ball the rest of the play - so if PLAYER A steps out of bounds and touches the pass first and bobbles it, then PLAYER B from the other team can regain possession or whatever is advantageous.
Player runs out of bounds, first to touch it, and catches it. If illegal touching is accepted here then it is a repeat of the same down from original line of scrimmage (as long as player has reestablished themselves inbounds).
To further your question to make this more thorough...
Let's take an actual ineligible receiver. Also let's make this real world so we don't have non-sense ideas that make it too complex. So a team lines up in trips to the right and the middle and the outside WR both line up on the line unbeknownst to them. The middle WR would be ineligible to go downfield as he is not the end on the line.
The NFL's definition is simply if the QB is in the pocket they have to throw the ball with some sort of reasonable chance of a completion. I know there is wording that states that it must be an eligible receiver but the broader definition is the spirit of the rule.
Scenario #1 QB hits middle receiver who is ineligible on a slant route on a normal pass play. Not grounding. QB not under duress and not trying to save yards throwing the ball away. For sure not grounding, definitely illegal man downfield or illegal formation.
Scenario #2 QB is about to get sacked. Throws the ball 3 yards over the head of the middle WR. This would be one of those plays, that if it happened in the playoffs would create a need for better wording on the IG rule. However I have seen ref sessions where this example has come up. According to the spirit of the rule this should not be intentional grounding as the QB was making a reasonable attempt at a completion. Basically the referee has to assess if the QB thought the receiver was ineligible or not. In this case it is clear that the QB would deem this receiver to be eligible. No grounding, again illegal man downfield.
Scenario #3 Team is running a screen pass. The offensive line lets the defensive line through and curls to block out to the right. The runningback runs his route to the wrong side. The QB starts back pedaling because he can't find the runningback. As the defensive line closes in on him there are several offensive lineman 3-5 yards downfield already. QB tosses the ball right beside one of their feet - beyond the line of scrimmage and near the player. Now this IS grounding as the QB under no circumstance could have thought the offensive lineman were eligible (doesn't matter if the throw was an accident). Since there wasn't an eligible receiver in the vicinity the defensive team gets to choose between illegal man downfield and intentional grounding.