Simply put, if you create a fanciful term you will be able to operate with greater reassurance that you will not face trouble in the future in relation to your choice of name.
Part of the problem here is the imbalance between the parties' respective positions. The article in fact admits that it is not users being confused that Instagram's app is related to their existing app, but the other way round. You might say that due to Instagram's size, they have the ability to instantly develop goodwill around a new app and its name (I am playing devil's advocate here to an extent).
If I was Instagram I would probably offer a small sum to Bolt.co to change their name on the basis that I could offer a sum now or know I have a deep reserve of budget for legal fees to argue that Bolt.co has not developed sufficient goodwill/reputation to be entitled to prevent Instagram's usage of 'Bolt'.
What? That is not at all how trademark works.
If Instagram wanted, it might have a shot at a protracted lawsuit, attempting to make it not worth Bolt's time to defend the trademark [0].
If that were to happen, though, I would hope that the suit would be dismissed with prejudice because in my non-legal opinion, any reasonable person would agree that Instagram is directly infringing Bolt's trademark in an area of business that is too similar for the average consumer to be able to differentiate.
[0] either by bankrupting them if they decide to defend it, or if Bolt can successfully mount a defense, it would require a large time investment by key employees like all the C-level executives, meanwhile Instagram (having more resources by far) is not impeded in its business nearly as much.
Like Apple?