This can be explained in this way. Sure, modernization means less unit of labour required for production, but this should not be taken as reduction in employment.
Growth in productivity, would definitely result in the growth of production level and income level. Growth in income will result in more demand of products and services. Growth in demand would also result in growth in supply. Therefore, it eventually results in more employment opportunities. Apart from this, various structural changes are also found in the economy. For example, the agricultural sector transfers labour to the secondary and tertiary sectors of the economy.