Kraken is one of the OG cryptocurrencies exchange. Maybe the oldest still around. They're in the business since 2013 or so.
Since then the Crypsy, Quadriga, Mt Gox, FTX, etc. have all come and gone (with their customers' money) but Kraken is still there. If they are on a scam, it's a really long con: a decade and a half of a con?
They're not "Binance big" but they do nearly as much volume as Coinbase, so 3 400 employees doesn't feel that surprising.
I don't think BlockFi did anywhere near the volume of trades Kraken is doing. As I understand it BlockFi was some kind of a "fire and forget" thing where you'd loan your crypto and get some yield in return. So very little things actually happening there compared to Kraken.
Yes, 900 employees was insanely huge for BlockFi. But I don't think 3 400 employees for Kraken looks that crazy.