This is not entirely true.
These people are passive.
It could also mean a trap.
Meaning it's a weak hand but the opponent is not folding because they are bad.
maybe the most reliable tell in all of live poker: when your opponent quickly bets exactly double the size of the previous street.
say your opponent bets $100 on the turn, and now the river brings in a flush draw and he quickly bets $200.
usually when players make a flush here, they pause to think about their ideal value size. and usually when they had a strong hand and the flush comes in, they pause to consider if they still want to even bet for value.
what hands bet quickly? bluffs.