Although they do have advantage of playing for longer it doesn't mean they will always win in terms of huge money (LetsGiveItASpin lost £100,000 euros last year 2019, playing deposit bonuses and live table games etc).
They may win £20,000 one stream but lost the previous 40 streams of £1,000 deposits meaning they are still down. There are loads of costs involved in streaming depends on the streamer such as Nickslots, he does random cash giveaways, 10% guessing competitions, all the charity donations, Tax on affiliate earnings, website costs, other stuff I can't think of right now. however if you did a yearly ins & outs 95% of streamers will still be down in deposits. which it would be double without the deposit matches, anyone can be an affiliate and stream pretty much now days.
Some affilaite partners are having max cashouts on deposit bonuses now which I won't mention who however the ultimate aim for deposit bonuses are intended for 1 thing only and that's to keep the streams going for longer to get their channel expand.
If Nickslots as he does stream 4 days a week and had a budget of £1,000 per stream without deposit bonuses it would be £208,000 annually so its just to keep costs down, keep the streams longer.
Then he also has his own costs on top as I mentioned above which include huge giveaways, charity work, tax on affiliate earnings, computer equipment it just makes sense to use them.
The UKGC has really messed up casino bonuses ages ago by taxing them so each average player hardly gets them anymore as the casinos don't want to pay any additonal tax costs for giving you an extra chance to win money. There are some sites that do offer reload bonuses but its very rare to get 1 now I don't remember the last time I had 80% bonus up to £240 @ Casumo its been years.
In terms of Max bet situation people love high rolling slots so some streamers may feel the urge to raise the bet to pull more viewers in, slotspinner has played on 5 euro average bet size for years which is a fair bet size.
I heard that Dunder has removed all deposit bonuses for streamers because people abused them (I don't know if this is the story I was told in a stream before.
Conclusion: Your right they do get slight advantage but not always win in terms of money, maybe better RTP and stream longer however even with a deposit bonus its still hard to win on the slots.