MOAB is a blast bomb and is very different design and for a very different use to the WWII Tallboys and Grandslams. The US equivalent to the Grandslam is the 30,000lb GBU-57B MOP.
https://en.wikipedia.org/wiki/Massive_Ordnance_Penetrator
There are also nuclear versions of this type of bomb to create even bigger 'earthquakes'.
It is a blast bomb, but it had the same effect as the earthquake bombs; it destroyed underground facilities.
In addition this was a good example of a strategic attack AND a "statement" by the US to other nations, like North Korea. It has had a dramatic effect already in both fields.
Totally agree, the US is back into the geopolitical arena with a big stick, much like Reagan was after peanut farmer Carter. Military force = political will * Means. If either is zero, like political will was under Obummer, this can quickly change if you have big Means, and the US with a $550bn defense budget (reduced by Obummer from $600bn) has. A few bad global actors need straightening out a bit and to wind their necks in now there is a new Sheriff back on the global stage, who is not afraid to use his 'Colt 45'. Trump's mental instability (like Tricky Dickies at the end of his presidency) is worrying, but the US has been there before and wiser council will be between him and his actions, including the 'big red button', this is thankfully because of their long history of strong democracy and institutions with their checks and balances, this is how strong democracies work.
Democracies are slow and cumbersome because we have wisely found out that collective brain power with different views and experiences generally make better decisions over a despot dictator's one brain who can quickly be either occasionally very right or mostly very wrong (Putin is a good example, especially with his invasion of East Ukraine). Hitler is an even better example of this which is why Churchill decided not to use decapitation as Hitler was the German's worst enemy! In the East his policy of not giving an inch of territory was particularly disastrous, along with the drugged up fantasist Goring thinking he could supply trapped armies from the air!
A big problem for the West is that Russia did not lose WWII and we need to contain their dysfunctional arrogance, without causing a nuclear war. Russia is especially dangerous (like North Korea) as they are both failed states with building internal pressures. Russia is suffering major industrial unrest (largely through non-payment of wages) but also political like the recent demonstrations have shown. What is especially dangerous is that Putin has placed his administration on eternal geopolitical expansion and success due to his economic failings, which his dysfunction state can't recover from, but now he can't afford to lose geopolitically.
We are in especially dangerous times, especially with a new mentally unstable sheriff in town!!!