Put simply, ‘strength’ is the ability to generate force.
If you have increased your strength (got stronger) you would have increased the ability to generate force, which is clearly measurable – 1kg became 5kg then 10kg and so on.
If the load remains the same, your strength has not increased. Sustaining the same force for a longer time (time under tension) is not an in increase in strength; that is an increase in endurance.
A lot of commonly recommended isometric exercises are not strength exercises by any stretch of the imagination. Avoid being confused by the two.