The description
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:
Series 1
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:
Series 2
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」:
You can even get 「言」 derived from 「舌」 with spittle drawn:
商
甲
後2.41.9
合集21631
Glyph origins summary
商
甲
甲499
合集30697戰國
金
中山王方壺
集成9735秦
簡
秦律十八1
睡虎地秦簡西漢
隸
老子・甲
馬王堆帛書
楷
「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」.
References: