I have read many times that the early medieval spatha was made of poor quality iron that couldn’t keep an edge

European swords from the late Roman period through to the end of the early Medieval period vary a lot in the iron/steel they're made from and their hardness. Some swords are all-iron (i.e., carbon content is below 0.3%, so the blade can't be hardened by quenching). Most are steel-iron composites, often decoratively pattern-welded, often with hardenable steel edges; some, but not all of these hardenable swords have been quench hardened. The smallest group are all-steel, and against some quench-hardened and sometimes not. Carbon contents of the edges of the hardenable swords are usually from about 0.4% to about 0.6%. Hardnesses vary from about 180VPH (unhardenable iron) to about 700VPH (about 60HRC); about 300-350VPH (30-36HRC) is common.

The hardest of these blades are harder than many (or even most) swords made today, or in the last few centuries, and would have excellent edge retention. The average sword was much softer, and would be considered sub-standard in modern times (where sword hardnesses of about 45-55HRC are usual, with some up to about 60HRC). The softest ones would have relatively poor edge retention - they would not stay sharp for long in use.

However, not staying sharp for long when in use is a quite different thing from not being able to be sharp. A wrought iron or mild steel blade, low carbon and not hardenable by quenching, can be sharpened to a fine edge, and will cut. It will quickly become blunt, but not many cuts need to be made by a sword before an opponent is killed or at least badly wounded. For an example of an unhardened mild steel blade in action, see:

• https://www.youtube.com/watch?v=SVSopH2RU1s

Such a blade can stay sharp enough for long enough to be useful.

that it was more for bludgeoning than for cutting.

There is no need of going to the expense of a sword for bludgeoning - it is simpler and cheaper to use a mace or a club. Swords were generally sharp (with the exception of purely thrusting swords, and modern military issue swords which were often only sharpened when at war (so that training would be safer, and in some cases so that the blunt swords could be used as less-than-lethal weapons for crown control)). it is rare for early iron/steel swords to be found in good enough condition for the sharpness of the edge to be judged, but when they are, they appear to have been quite sharp. Where the effect of swords is described in literature, they appear to be sharp. For example, when Beowulf fights Grendel's mother, he is surprised when his sword, Hrunting, fails to cut her:

For mighty stroke

he swung his blade, and the blow withheld not.

Then sang on her head that seemly blade

its war-song wild. But the warrior found

the light-of-battle was loath to bite,

to harm the heart: its hard edge failed

the noble at need, yet had known of old

strife hand to hand, and had helmets cloven,

doomed men's fighting-gear. First time, this,

for the gleaming blade that its glory fell.

No failure of the sword, per se, but Grendel's mother's mighty magic! He then picks up an ancient sword (possibly even an old Celtic or Roman period sword, and therefore less likely to have a hardened edge), and uses it:

'Mid the battle-gear saw he a blade triumphant,

old-sword of Eotens, with edge of proof,

warriors' heirloom, weapon unmatched,

-- save only 'twas more than other men

to bandy-of-battle could bear at all --

as the giants had wrought it, ready and keen.

Seized then its chain-hilt the Scyldings' chieftain,

bold and battle-grim, brandished the sword,

reckless of life, and so wrathfully smote

that it gripped her neck and grasped her hard,

her bone-rings breaking: the blade pierced through

that fated-one's flesh: to floor she sank.

Bloody the blade: he was blithe of his deed.

A later tale, Kormac's Saga, describes Cormac's unlucky loss in a duel to first blood:

Bersi struck the first blow, and cleft Cormac's shield; Cormac struck at Bersi to the like peril. Each of them cut up and spoilt three shields of the other's. Then it was Cormac's turn. He struck at Bersi, who parried with Whitting. Skofnung cut the point off Whitting in front of the ridge. The sword-point flew upon Cormac's hand, and he was wounded in the thumb. The joint was cleft, and blood dropped upon the hide.

Cormac breaks the tip off his opponent's sword (damaging his own borrowed sword in the process), and the sword-tip hits and cuts his thumb. After having been used to destroy three shields, Bersi's sword-point was still sharp enough to wound through incidental contact with little force behind it.

There are two additional reasons to think that these swords were, typically, sharp. First, clothes were commonly worn in Europe. To wound the person wearing the clothes, it is important that one's sword can cut through cloth. This needs a sharp sword.

Second, swords from the Roman period and early Medieval swords often have snug hilts with guard and pommel that will sit close against the top and bottom of the hand. Hilts like these encourage the sword to be held at close to right angles with the forearm (in what is often called "hammer grip", with a straight or almost straight wrist). This grip works well for fighting with a shield, allowing effective slicing draw cuts. Draw cuts are often praised as the most effective sword cuts (e.g., by 19th century European writers on military swordsmanship British, and writers comparing British swordsmanship with Indian swordsmanship (and we can note that the talwar (or tulwar), a common Indian sword, has a similar draw-cut-friendly snug and supportive hilt)). However, they are poor for bludgeoning, since the slicing movement reduces the bludgeoning effect. These swords work well with draw-cuts, and therefore probably had sharp edges.

For some interesting further viewing:

• A sharp iron sword (and African sickle sword) being used for cutting: https://youtu.be/IU3bcnxx18k?t=291

• The making and testing of bronze and iron swords: https://www.youtube.com/watch?v=V28ItY0K9ts (but not with challenging targets)