The 2501 could not be sold as a new tractor in 2018 w/o meeting tier 4 final standards.
Yes, it absolutely meets tier 4 final standards. But those standards require a different level of emissions for tractors over 25 horsepower. So yes, it's compliant with the standards, and no, it doesn't have the same emissions controls as the larger tractors (the 3301 for example). This can be seen at this link
Emission Standards: USA: Nonroad Diesel Engines, in table 2 a bit over half way down the page. Tractors 11-25 HP can emit 6.6 grams of CO per kWh, where tractors 25-50 HP can emit 5.5. For nitrogen oxide, 11-25HP can emit 7.5 g/kWh, 25-50HP only 4.7g/kWh. Particulates look to be the biggest difference, 11-25HP can have .3g/kWh, 25-50HP must be 0.03 g/kWh, so ten times less.
Bottom line is that a tractor in the 11-25HP class generally doesn't need a DPF to meet Tier 4, and a tractor in the 25+class generally does.
So, you need to either buy a tractor 25HP or less, or buy a tractor 25-50HP with a build date before 2013-ish.
In a perfect world you'd be able to buy an
L2501 and then do some aftermarket tuning to produce more power. Presumably that's illegal or discouraged though.