First of all, there is no E8018B3. There is an E8018B3L. And the L (low carbon) is what is responsible for it being 80 instead of 90. The electrode was developed for the express purpose of welding those alloys when PWHT is impossible or highly impractical. The L would also be integral to minimizing preheat. But you need to make sure they comply with the code.
Per your info the electrode was E8018-B3L, which is the low carbon version of 2.25Cr-1Mo electrodes. The E8018-B3L has a maximum of 0.05% carbon and an 80,000 psi specified minimum tensile strength. The E9018-B3 has a specified carbon content of 0.05% to 0.12%. and a specified minimum tensile strength of 90,000 psi. In contrast, the SA213-T22 tube material has a specified carbon content of 0.05% to 0.15% and a specified minimum tensile strength of 60,000 psi.
So, the minimum tensile strength of an E8018-B3L is still much greater than the T22 tube and there would not be a strength concern. The low carbon electrodes are a good choice where postweld heat treatment will not be performed because their lower carbon content reduces the hardenability of the weld deposit and potential for cracking.
Not performing the preheat presents several issues. Metallurgically, there should not be a problem with the microstucture in the heat affected zone of a tube weld. Preheat is intended to slow the cooling rate to minimize the potential for martensite formation. However, in a thin-wall (less than ~3/8"), small diameter (less than ~4" OD) T22 tube with 0.15% maximum carbon content, the cooling rate is not rapid enough for martensite formation and it is not usually a problem. I've examined the HAZ microstructure in new T22 tubes that were overlayed with nickel alloys while water was pumped through the tube. This would represent a worst-case cooling rate, but the actual HAZ looked fine with no martensite. However, when welding on old tube it is good practice to preheat the tube to drive off any moisture that is present on the surface or in any corrosion products. The other issue is Code compliance. Per Table PW-39 in ASME Section I, for a circumferential butt weld in a P-No. 5A tube to be exempt from postweld heat treatment the following conditions must be met:
(1) a maximum specified chromium content of 3.0%
(2) a maximum thickness of 5/8 in.
(3) a maximum specified carbon content of not more than 0.15%
(4) a minimum preheat of 300°F
So, I'll agree it may be metallurgically acceptable, but from a workmanship and Code compliance standpoint it is not acceptable.