At the table's summit and against the league minnows, Friday was supposed to be the night Arnold became the second coach in history to achieve the feat - and with a superior win ratio to the only other centurion Ernie Merrick.
But the 54-year-old didn't count on the Mariners' pace and intensity that neutralised his Sky Blues machine, nor the goals from Hoole and Jake McGing that sealed his side's first loss in nine months. It was the former Mariners coach's first at Central Coast Stadium since departing in 2013.
Last campaign, it took 20 rounds for a team to defeat Sydney, when Western Sydney became the only side to down the would-be Invincibles in February's derby.
The Mariners managed it in six rounds, a result that propels Paul Okon's youthful team into an unlikely sixth spot for just the second time in two years.
From the outset, they prodded and poked holes in Sydney's rhythm - or perhaps the champions did that to themselves, such was the disunity at times.
The hosts' near two-thirds of possession produced chances and it didn't take long for their counter-attacking streak to yield a fine team goal.
Danny De Silva set up the move after Luke Wilkshire turned over the ball on the left flank, trading passes with Connor Pain and Asdrubal.
Back in possession, the Spaniard anticipated the on-rushing Hoole and fed the former Sky Blue and Newcastle Jet who, after outrunning Michael Zullo, had only to maintain his composure to evade the wrong-footed Andrew Redmayne.
Sydney entered the break on the back foot but a stormy-faced Arnold knew his relentless squad's uncanny knack for a late fightback - it had been 19 months since his side had conceded more than one goal and lost a match.
It became clear they might not make it to 20 after McGing headed in a Hoole corner, Josh Brillante guilty of losing sight of the ball.
That lit the fire in an already feisty affair, and Wilkshire earned a yellow for a tackle on Pain before bandaged-up skipper Alan Baro was given his marching orders six minutes from time after a second booking.
Hoole came agonisingly close to netting a third but the result was already set in front of nearly 8000 jubilant fans.