Stat Boost Guide, wrong math?

Yes, they're wrong.

However, it is more than a 5% boost.
A 50% boost to attack at 10% chance is a 5% boost to your damage per turn.
However you're also getting a 50% boost to defence, which "buys" you 5% more turns.
So the overall boost to your attack is effectively 1.05 x 1.05 - 1 = 10.25% boost.

Further, in the 10% chance that you got the 50% atk and def boosts, you're now sufficiently stronger to have more time to look for additional boosts. So the overall bonus may actually be a bit closer to 15% (probably more like 11% though) - even though that's definitely NOT what they meant.

The guide leaves a lot to be desired, tbh. It briefly touches on the details, but those details are really what is important.

A 50% buff that takes a 0% win chance to a non-zero percent win chance is really a much larger than 50% buff. In reality, it's a 10% boost to your win chance AND leaves you with a stronger pokemon alive after getting that win. However in the contrary to this, if it's not your last pokemon then that 0% win chance going to dealing even less damage to this pokemon could still be a worse scenario.

They also say how much you win or lose by doesn't matter. This is completely untrue. PvP is a 3v3 battle. How much you win or lose by DEFINITELY matters. Especially when you have a top-tier pokemon getting a 50% ATK and DEF buff in the winning state. It may be strong enough to beat the next pokemon too. Winning by a lot matters. Losing by a lot matters. If going for the buff and missing it means you applied too little damage to the current pokemon for your next pokemon to be able to beat it (or be able to beat it quickly enough to avoid taking a charge move, or to give the opponent a shield advantage plus the first pokemon KO, etc...) then it's a terrible decision.

I think ideally you want to look for buffs in your first or second matchup, so when you get them you can carry them onwards. You also especially want to use them against shields, AND while you still have shields. So you can CHOOSE to shield a pokemon that has received the buff to keep it out there SUPER long. If it didn't get the buff - you can save your shields for your next pokemon. i.e. you get choice, and that choice has value.

Here is how I see it, both atk and def get a 1.5 multiplier resulting in 2.25 multiplier to the TDO. Since it's only 10% of the time, taking the average results in an average 1.125 multiplier to the TDO. This is still too high since you don't get the boost from full health, let's assume on average you get the boost with half health, the result is indeed closer to an equivalent 5-10% boost to the TDO.