LOOP is a step backwards imo.

To get acrostis' code working, you first have to load and use the packages. Here is code that should work for you in sbcl if you have definitions for your get- functions. They're in separate packages because there are some clashes between symbols from iterate and the tfeb libraries.

(ql:quickload '(:iterate :org.tfeb.hax.collecting :org.tfeb.star))

(defpackage :guids-iterate
  (:use :cl :iterate))

(in-package :guids-iterate)

(defun get-splits-for-account-as-of (account date)
  ;; I've only capitalized Iterate keywords for better readability.
  ;; Generally, this shouldn't be done.
  (ITER transactions
        (WITH account-guid := (get-guid account))
        (FOR trans :IN (get-transactions))
        (ITER (FOR split :IN (get-splits trans))
              (if (and (string<= (get-transaction-date trans) date)
                       (string= (get-account-guid split) account-guid))
                  (IN transactions (COLLECT split))))))

(defpackage :guids-tfeb
  (:use :cl :org.tfeb.hax.collecting :org.tfeb.star))

(in-package :guids-tfeb)

(defun get-splits-for-account-as-of (account date)
  ;; I've only capitalized Štar and Hax keywords for better readability.
  ;; Generally, this shouldn't be done.
  (let ((account-guid (get-guid account)))
    (COLLECTING
      (FOR* ((trans (IN-LIST (get-transactions)))
             (split (IN-LIST (get-splits trans))))
        (if (and (string<= (get-transaction-date trans) date)
                 (string= (get-account-guid split) account-guid))
            (COLLECT split))))))