We are, but as Colt remind us, he only became a General later on. At the time, he was a Major of the AFFC/SLDF and a recipient of some prestigious Star League award.
Which poses a problem that we can't really reconcile. On the one hand, his award and some of what Victor says to him (I don't remember it clearly myself) hints that his mech was rebuilt for him as a reward and an honor. This largely makes sense, especially in the traditional neo-feudal atmosphere of BattleTech (that may or may not have largely evaporated since the 4th Succession War) in that it's ultimately Archer's mech (why he's able to take a better than top of the line machine with him into retirement). But, if it is a reward and an honor, as has been pointed out, why not refit the MPLs too? After all, while Clan ER LLs are better than IS ones, Clan LPLs are dramatically better than IS MPLs, and that refit would have made his Penetrator insanely dangerous. Or why not upgrade it in some other interesting way?
My theory is that, sure, it was an honor to even have a mech at all, but they were still on Huntress, hundreds of light years from home and in the midst of a pile of angry Clanners who could have easily fallen on them and wiped them out of existence given the political will. More over, while Archer fought well and bravely and for a noble cause, he did so for about five minutes. Would it have been right to upgrade his mech at the expense of those who fought nobly and bravely for weeks and months?
A quick look at FM:U hints that Huntress yielded up regiments of Clan tech. The ELH are still about 50% Clan even after five years and a combat deployment to St Ives, the 1st Royal is famously 100%, and even the surviving battalion of Kingston's is still about 33% Clan even after a stint in the Periphery and some (presumptive) poaching by other CCAF commands (there are a lot of assumptions in that one, but I'll stand by them in an appropriate thread if called upon). There's no reason to believe that there weren't enough Clan weapons to refit Archer's mech (or just replace it, as Paul Masters did and doubtless scores of others less famous). So why not?
Honestly, I'd guess that it wasn't as high a priority as Victor may have made it out to be, at least not for the quartermasters who actually had charge of the operation. A single warrior who's not even combat ready and going to muster out soon anyway (I don't know if that was known, but if it were it would be something I'd think about if I were a quartermaster)? Sure, I'll throw you a bone, but someone in universe has to be aware that the Clan MPL is a strong contender for the single very best weapon a mech can mount (behind perhaps it's big brother), and if they wanted to allocate them to combat ready warriors, or ones who'd fought and bled through months of Hell and were hot to stick it to the Jags and their ilk, who could blame them?
I don't think the solution is knowable (excepting perhaps a future publication, or one I'm not aware of, or I suppose an ask the authors). For me, I'd probably just let the mech be under weight. But, if my opponent wanted to add a sink or two, or more armor, or something else non-intrusive (AMS ammo? under the old rules reasonable, if not under the present rules) I don't think I'd make a fuss about it.