Commit 1ef147cdc5bab0a84bb30f3af1b8355c38fe433e on 19 April 2019 for bugs 396225 and 392305 showed ALL accounts when an import needed to map to an account, but the information in the import was wrong and led to the desired account not being listed. I think the commit went a bit too far, as now even investment accounts are listed, and these are inappropriate for importing any account type except investment. (I'm hoping no institution gets this wrong, but only sometimes mixing asset with expense types.) As this is only a minor inconvenience, it might be reasonable to make this a wishlist. The original commit had a comment that one possibility was a UI to allow selection of what type of accounts should be shown if the desired account was not on the list originally presented for import. In addition - if the list is being shown to choose the brokerage account to use when importing an investment account, that investment account should certainly NOT be included in the displayed list. Note commit 1ab18d6b13b20af947a3c43e9a67a1db1e97d430 from bug 363069 in 2016 apparently also addressed this issue, hopefully not relevant now.
Actually I just discovered that you CAN import checking transactions into an investment account. I was importing an investment account (KMM OFX online import) and accidentally selected the investment account instead of the brokerage account for the cash part, and for dividend transactions, the "deposit" became a "buy shares." Once I realized what had happened, when I edited the Dividend transaction, and correctly specified the brokerage account (instead of it being the same investment account) the "buy" transaction (appropriately) disappeared.