I am wondering how they pulled it off.
Luck
They've also mined 5 blocks in a row in 19 minute sa bit earlier 829,215 to 829,219
Foundry mined 4 blocks in 4 minutes, 828,838 to 828,842 and earlier got 6 in a row , 828,822 to 828,827.
Nothing special, happens all the time.
Viabtc mining 4 blocks in a row is more of an event since they have barely 10% of the hashrate.