To create 言 - Wiktionary, mustn't you add more than one mark to 舌? Sorry for not being able to visualize abstractly!
言 was created by adding a mark to 舌
should really be
言 was created by adding one or two horizontal marks to the top of 舌
Details aside, it is a fairly accurate character component arrangement description. For it to make sense, you have to first determine the shape of the character components that it's talking about.
Wiktionary's glyph evolution table of 「舌」 doesn't show enough examples of 「舌」 in oracle bone script for the description to immediately make sense. The shape we have today is originally inherited from Series 1, shown below:
Crossed out samples are ones where spittle is also drawn, which doesn't survive today.
The confusion arises because Wiktionary is only showing a sample of 「舌」 from Series 2 below:
It doesn't really matter which shape of 「舌」 that 「言」 is derived from, 「言」 is still made up from one or two horizontal marks added to the top of 「舌」:
「言」 derived from Series 1 「舌」:
「言」 derived from Series 2 「舌」:
You can even get 「言」 derived from 「舌」 with spittle drawn:
Glyph origins summary
「言」 (to speak) was originally comprised from one or two horizontal marks 「一・二」 added to the top of 「舌」 (tongue). The shape which survives today is made up from two horizontal marks 「二」 on top of a 「舌」 shape coming from Series 1.
「舌」 is no longer evident in 「言」 due to some minor shape corruptions to the forked tongue shape occurring post-Qin. Compare 「音」, which is actually comprised from 「言」 with a differentiating mark added inside of the 「口」 part of 「言」.
In 甲骨文 form, you can see it clearly.
Please see 舌 and 言 in the right side of picture in this website.