The letter said cryptocurrency income, it didn’t said mining. Most probably referring to trading income.

If you declare, you pay (at most) 25% of the income as tax.

If you no declare, and LHDN find out, you pay 25% of the income, plus 80-300% of that, so effectively govt will take up to 75% of your profits. And it is pay first talk later, so good luck hiring a lawyer when you need to hand over 3/4 of all your gains, especially if you already "reinvest" it and the market turn into another years long bear market.