Because the money goes straight into supporting the bot and paying for the costs involved in the bot, they are not for profit, or for Expert's work on the bot. It is a gift to help with the costs involved, and in turn, Experts rewards the donations with additional functionalities.
Given all the money goes back into the bot anyway, it wouldn't result in any taxable profits, so no tax is avoided, even if they were considered fees.
In addition, since Experts is in Singapore, the tax laws applying to him are almost certainly different to those you might be thinking of in relation to your allegation.