dredknightbonus attack - at level 30 a hero will get a bonus of 4 attack which is about 13.32% damage increase for the whole army.
bonus to creature stats - at level 30 a hero will grant 15 attack and 15 defense to a specific stack which is 49.95% damage and 49.95% defense boost

This is not correct calculations for 3.33% bonuses at MMH5.5 mechanics. It's closer to original 5% bonuses.
The general point is that all bonuses are calculated for BASIC damage, not current.
For example (in 3.33% mechanics), +3 attack give +10% of current damage, but only if the difference is 0 (for example, 12-15 attack vs 12 defence). If the difference is more or less than 0, bonus will be other. For example, 42-45 attack vs 12 defence give 210% damage vs 200% damage, or only 5% (210/200 = 1,05) bonus damage.

In short, real bonuses are less than basic. Having 150% number of creatures is much stronger than having "49.95% damage and 49.95% defense boost", even without destructive spells.