Well, from the user who shared his bonus offer from stake.com I don't think he said anything about him being inactive which may have attracted such jaw dropping bonus to him. So it's just a random actually. But in all, it's better to stay active enjoying the games in Stake platform while hoping you could be the next in line. Because you can still stay inactive and get nothing since it's something that seems to be random.
Gambling platforms don't usually give users who are inactive bonus, they only do that to those who are active using their platform, though they do something that when they noticed inactivity on a user's account after gambling for a while, they present bonus base on their previous activities and wager made and encouraged them to be active and continue with gamble again on their platform.
Gambling platforms constantly track all their active and inactive users. If users who used to gamble a lot and complete a lot of wagers quit gambling, they are offered various types of bonuses to bring them back to the platform. However, they do not offer bonuses to all inactive users. And they constantly offer various types of offers and bonuses to active and potential gamblers so that they are more motivated and spend more time there by gambling. This is their business and it is natural that they will always try their best to grow their business.