I am new to OP cleaning and was wondering if Dirt Assassin is good for encapping or is it for HWE only.
Yes.

I use DA as one of my mainstay chemicals. I often add it to my other encap sauce, peroxcellent.
It forms a film rather than a crystal and, as such, is slightly harder to vacuum away. But even so, it is still non-sticky so it does not attract more dirt. So you can use it either way.
for 'encap' I suggest you use an absorbing type of pad/bonnet rather than just a tuway or fiber pad.
I do not always do that

but I also hardly use DA alone. But if I did, I might not pad it out anyway unless the $ was there to do so.
Bottom line, it is an awesome cleaner but so is Hot Knife. Which can be encap only.